Agree on the gear vendors overdrive for the cheapest least involved route. 2nd to the NV4500 swap probably cheaper but more involved.
americanpowertrain.com
https://www.gearvendors.com/hrgm4sm.html
http://www.gearvendors.com/wp-conten...nual_Trans.pdf
Just an example, definitely not the cheapest route nor intended to be, my swap is listed below bear in mind it's all new parts:
T56 Magnum + install kit + hydraulics (includes driveshaft, crossmember) $4713.18
LS7 Magnum clutch kit + Flywheel $541.24
Ebay bellhousing $276.78
=$5500 + and I'm doing it myself so that's no labor involved either.
I'd go gear vendors if I were you, less involved, easily reversible, resale.
If you're dead set on swapping a different trans, NV4500 TKO500 TKO600 T56.