I did this changeover two years ago in my 91 5.0Simple Me said:
I bought a GM 350 crate motor and have put 35,000 miles on the new engine. I don't know why you would want to go from a TBI to a carb.
The bellhousing will match up. Go with the GM crate motor, it has a 3yr. 50,000 mile warranty. Don't rebuild buy new it's cheaper than rebuilding.
I ran Valvoline 10w30 for the first 5,000 miles and then switched to Mobil 1 10w30 after that.
Good Luck.