The later transmissions (69 up)use a larger front bearing and bearing retainer therefore have a larger center hole. The center hole doesn't support anything, it is for clearance of the bearing retainer. The 55-72 truck bellhousings mount on the frame cross member. If you use a later one you will need a rear mount.
I'm fairly sure the clutch gear is longer on the T5 and needs to be modified. Others in this forum have extensive experience with T5 installations.
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