Gear Vendors or swap to better tranny?

Hello, fellow Mopar freakazoids.

So, the 318 in my 71 Demon has minor perf. upgrades, but am preparing for more. I have a 3 spd 904 Torqueflite and was thinking of adding an GV unit instead of upgrading the tranny. The price for the GV unit is $2850 while a 5 spd performance tranny from transmissions.net runs around $2600. The rear end will be upgraded from 7.25, to 9 inch or so. My trusty mechanic recommends upgrading the tranny will be cheaper, but I don't see how when the tranny price is close to the GV price and labor hours and modifications seems much lower installing the GV, not to mention the GV allows for 6 speed and gear splitting. I am assuming this question has been asked a bazillion times?