NuCKinG FuTS said:
its a jeep 258 4.2 its carb'd
There is alot you can do to that motor to warm it up a bit, make some more HP, Torque, etc.
First thing I would do is check out what Clifford has.
http://www.cliffordperformance.com
Probably go with a more free flowing intake and exhaust (they have intake manifolds to swap in a 4 barrel carb) also go with a header.
Next step would be to swap in an RV cam or a horsepower cam. In my Cruiser I run a cam with a 273 degree duration. Gives me more horsepower, and more torque. Moves the power band into a useable area, without sacrificing low end torque.
After that, I'd look at shaving the head and installing some oversized SS valves.
After that, it's a choice of swapping the motor, or building a Stroker. I think you can do all of the above things for far less money than what you would have to pay someone else to swap in, adapt, and wire the Northstar V8 you mentioned.
If you can do most of the work yourself, and/or have buddies that can help you who know what they are doing, you can do a V8 swap pretty inexpensively, but at that point, I would consider a TBI 350 with an SM4xx and a Dana 300 T-case.