You could expect to pay $400 to $1500 for a NV4500 depending on where you get it and the condition. Mine was a trade deal so no money out of pocket except for the $25 tcase and cost of the AA kit.
If you don’t mind not having overdrive, your TH350 is a great trans and work well behind a big block. A TH400 better but a TH350 is sufficient. If you would like to step up to overdrive a 700R4 or a 4L80E would be the best swaps in automatics. There is some wiring required to retrofit both and a computer needed to run the 4L80E. If I had gone the automatic route I would have gone with the 4L80E. An Allison 1000 would be idea but that requires body modifications and a body lift to bolt one in an old GM truck. Neither of which I am a fan of. The best way to figure out what it would cost is to figure out which recipe you want to go with then start pricing the parts than double the parts costs you come up with as something always comes up during the project that blows your budget. These projects are never cheap. My project took 3 months and it still isn’t truly done while some gremlins are being worked out.