I doubt that the T-4 has a much lower first gear than the original 3 speed or the T-5. I think you could do the T-5 conversion much cheaper than buying the T-4 and then having to replace the rear end gears. You could also look into the factory overdrive that was available for a 65.
Fred

1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine