The newest Wranglers already have a coil suspension, but reengineering an older one to that would take serious money. If you're looking to improve the ride, shoot some grease between the leaves and replace the rubber bushings with polyurethane. Use passenger all-season tires instead of off-roaders or light truck tires.
I'm not sure where I saw them, but I've seen lowrider spring shackles that bring the Wrangler down about 1.5" and it's surprising what an affect that has on the handling. Those might've been a custom job, but it isn't too difficult to do.
You can bolt a Chevy engine into anything you want. There's adapters out there to fit the LS series engines into everything up to farm animals.
You want my recommendation, go backwards: get the straight-six 4100 engine in there. Slow, but monster torque. Just watch out for head gasket failures and you're good to go.
