Open,that's good, you can do just about anything. It's all been done before. Vendors have kits for all of it. Not sure why you need to replace the whole thing if diff ratio is OK. Anyway, you need to figure out what you want 6 or V8 and what tranny, You can by the motor and tranny from a running truck to be sure it's OK. Then get the obligatory cross members to mount motor and trans. Modify the driveshaft length to suit. Auto is much easier to install and cheaper to rebuild. Lots of info on this stuff here and web. A SBC with a 700R4 would be a darn good choice. Don't make it harder than it has to be.
