I used the front half of the GTST manual prop, as that has the right fitting into the GTST gearbox
The front of the propshaft is a push fit into the gearbox, so few mil either way will not be an issue
As for the diff you'll need to ask others as mine will not be making 500hp, but I suspect it wouldn't last long
(assuming the drive shafts and hubs don't break first !)