cummins with a 12-bolt

[79GMC_OLDS]

ok heres the deal...i have a 79 gmc 1/2 ton 2wd and im putting in a 5.9 cummins and an allison at540 from a school bus. With this combo there should be 600-650 ftlbs at the wheels. The allison is a non od 4 speed.

So with no od , 28" tires and a max rpm of 2700 i need to run a high rear gear for driveability. I have a 2.73 open 12-bolt truck rear end to use. So question is... what do i ned to make the 12-bolt hold up? I cant run a dana 60, 70 or a 14 bolt gm because of gear choices. I was thinking posi so at least the torque load is split between the 2 axles. Does anyone makea 2 series posi? Will the axles and gears themselves hold up or should they be upgraded as well?
thanks