I have to disagree entirely.
You are free to do so, I'm just giving my perspective based on my perception and experiences
I am not sure you are reading his full criteria. He wants a truck with added gas mileage for not so much money and be used as a street/farm truck.
I read the OP's full criteria and no where does he say "added" gas mileage. His words were "reasonable" gas mileage. Considering that "reasonable" isn't exactly defined, we can't simply assume "reasonable" equals "added". It's easy to assume that he wants "added" economy, considering we would all like "added" fuel economy, but I think the OP maybe smart enough to figure out that added economy probably isn't possible if he wants 400hp. My guess is he probably doesn't want to get pummeled at the pumps either. It's a logical view to have if he understands that more HP equals more fuel is gonna be used. Now having said this, if his build is done just right, I don't believe he's going to get "terrible" fuel economy, but in my opinion, to stay within the ball park of what his fuel economy is now, he needs to increase the displacement and that means Stroker
Next in his original post, the OP says he wants to do this for "not a lot of money". Now, considering that he lists off a few parts, he never really specifies what he is willing to spend. So we don't really know what "not a lot of money" means to him. For all we know, he may have tons of money and is a tight wad, or he's just another working class Joe with 2.5 kids a wife, a dog and a ton of bills. All we do know about the guy's finances is what he tells us, and later in the thread, he says the stroker isn't within his budget. But's thats usually the case with anything we may want. Sometimes the things we want the most aren't within the budget and thats why I gave a few alternatives.
To all of THIS COMBINATION, I still contend-"no way".
For the OP, maybe, maybe not. It all depends on how much the Op has to spend, and what he's willing to compromise on.
Now, let me tell you that it's quite possible, because I've done it. I got a 5.9 in my Dakota now, that pushes close to 400hp and will see 20 mpg. From the cost perspective, I spent less on my engine then if I were to use the original 318. All I did was swap engines, I added bigger injectors and a big turbo. Thats it. My engine didn't cost much, and heres the best part, the engine wasn't rebuilt. It's got about 160K original miles on it. Total cost for the HP upgrades was under a grand. So how did I do it?? I stuck a Cummins under the hood. Yes my story is a bit deceiving because I spend a small fortune on numerous other parts like the drivetrain and transmission, but the point is anything is possible if you keep things in perspective including your personal finances.
A 200 shot of nitrous on a stock motor might hasten the end of that motor especially w/o the proper essentials for running nitrous. Will the op be cognizant of fuel pressures or timing take out or atmospheric conditions or burn gauging or intake temps?
I'm not sure you understand the meaning of sarcasm. A 200hp shot of nitrous will blow the pistons out the oil pan the first time you hit the button. The bottom line here, nothing is cheap and theres no quick way to make big horsepower (unless you live in a manson and fly your personal jet to polo games)
Sure you can build a 500 hp 318 but do you want this as a street/farm truck? A 500 hp 318 and the word "cheap" is an oxymoron. I think it a totally unreasonable quest. The only part of your post I concur with for HIS reality is lowering his expectations but even to 300 rwhp is not enough.
But he wasn't asking for 500HP, thats a huge exaggeration, he was asking for 400HP, thats a huge difference! If he were asking for 500HP, I'd say thats a lot to ask from a 318. 400HP is about the max I would even consider, and HP gets exponentially more expensive, the higher you go.…
A typical stock 318 is good for about 230HP (And yes a 200HP shot of nitrous would get him in the ballpark…..once!) So he needs to get at least an additional 200HP out of his engine. I sarcastically stated Nitrous will do it…..but don't expect the engine to survive. I pointed out the different ways to get there, and the way I'd personally get there is go Stroker for the larger displacement which is actually a less stressful way to get big power out of a smaller engine. However not knowing how much the OP has to spend means he may have no choice but to lower his HP goal. 300HP is far more reasonable and maybe closer to his affordability. Remember, the 318 is rated for 230HP(net) All the OP needs to do is make a 70HP increase.
He is asking way too much as he does not need hp as much as he needs torque.
As for torque…Yes, he needs as much torque as he can get, but there is an inseparable relationship between Torque and HP and can be clearly seen by the formula;
TQ X RPM / 5252 = HP
Now our OP wants to get 400HP out of his engine. So based on the formula, there are an infinite number of combinations of TQ & RPM to reach 400HP. He can use a 4 cylinder, a big V8, or even a diesel, so long as the combination of torque and rpm equals out to be 400hp. (Of course if you were using a 4 cylinder, it would need to produce a lot of torque, otherwise it would have to rev at insane rpms to be at 400hp) Now our OP can try to build the engine for torque but the only proven ways to build an engine for torque is thru greater displacement and/or improved volumetric efficiency (Thats forced induction) and yet there will still be limits. But if you're building a gas engine for torque, you're really building for HP anyway because of an interesting mathematical phenomenon found in the formula. An engine turning at 5252 rpms is making the same horsepower as it is torque. For example, if an engine was making 300hp at 5252 rpm, it would also be making 300 lb-ft of torque at 5252 rpm. That would also mean that all engines that run below 5252 RPMs tends to make more torque than horsepower and all engines that run above 5252 RPMs tends to make more horsepower than torque. This is why a diesel seem to produce more torque and why gas engines seem to produce more horsepower. Most diesels aren't even capable of rpms above 5000 rpms while gas engines tend to come alive over 5000 rpms. Because of this curiosity in the formula, if you compared a 400hp gas engine to a 400hp diesel, the gas engine running above 5252 rpm would be making less than 400 lb-ft of torque, while a diesel running below 5252 rpm would be making more than 400 lb-ft of torque. So to get back to the question, with a gas engine, building for torque "is basically" building for HP there is no way to isolate each and there no way of getting around it.
Ed