[Capri306]

It's more than likely a low-compression setup, with garbage heads and cast rockers. You're going to be hard-pressed to make decent horsepower out of it, but then again since it's in a "tank," as you put it, it's going to need more low-end torque. If I were trying to scoot in a heavy car, I'd be looking to put in more cubic inches. There's no replacement for displacement, and this is no exception. To that end, I recommend a 351W from a truck, with an aftermarket intake so it'll still fit under the hood. Finding a 351W isn't hard, and it's a drop-in for most full-size cars that came with 302's. As far as the C4, it will fit just fine as long as the cross-member is willing to cooperate. I'm pretty sure the driveshaft will work, but even if it doesn't, getting a custom driveshaft isn't expensive.

Actually, no, scratch this entire idea. You wanna blow off someone's doors? Do it right: big block, baby! Screw this small-block based stuff. 600+ cubes will make that mofo move like no other.