Swapping 4.3 for lt1

I'd say someone already has figured this sort of swap out, and likely has posted an in-depth guide on a dedicated Chevy truck forum. This site limits detailed responses and we can only insert one picture per post with no text formatting, so it's frustrating trying to post a "how to" of any sort. So for that reason, I'd suggest finding yourself a good ol Chevy truck forum and seeking your how-to there. The only other concern you should have is what your states emission's laws are, as long as your transplanted engine has a working smog-compliant exhaust and working OBD port, you should be good.