Step 0 for both of you. Grab your year's factory service manual.
RC96SLT - one thing that helps is that the 1996 was OBDII, so even though a 5.9 was not a factory option, you can pick up a V8 ECU, a 5.9 and a 46RE, add the two injector wires from the engine harness connector to the ECU, and go. Past that, call a tuner to get the max out of that 5.9. The 5.9 will need a Dakota oil pan, so ideally you find a Dakota donor for it; but if not, the pan can be bought.
And as mentioned, it can be stroked and bored slightly to 408ci/6.7L.
Define "biggest". The "biggest" engine to ever come in a Dakota is a 5.9
The "biggest" V8 Dodge ever made was the 440, and I know a guy sticking one in his Dakota
The "biggest" Dodge engine made are the V10s. Two kinds exist. The old gas hog, big torque Magnum truck engines and the exotic Viper V10s. I don't know of anyone who had stuff either into a Dakota, but I do know the magnum engines have very little performance parts available, and the Viper V10s are pretty pricey
The "biggest" engine used in any Dodge in terms of size and weight is the Cummins 6BT turbo Diesel, and believe it or not, it will fit under the hood of a Dakota, but you'll need a really big shoe horn and about 55 gals of Vaseline to squeeze it in. Ask me how I know
How fast you wanna go? Money is power, big money is big power. It's a matter of configuration. You can put a old school 426 in it, but you'd have to put additional frame, suspension, trans, and body mods on it. Splicing PCMs on not intended engines is above my pay grade. Just my opinion, if you do that? A whole host of changes would have to be made to the Valve-train, bore, and cams. Upgrades to cooling cuz it's naturally going to run hotter. A quality FCA mechanic is going to have to tune the thing, through the PCM.