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
