Everyone getting hung up in the rvnet fodder of "ratings."
OP what is your plan here? You got a pristine low mile grampa truck that you want to hot rod into a towing machine?
You get a free engine and Allison and it's taking up space in your shop?
You're bored and want a "project?" "Projects" don't usually make good tow rigs.
Have you added up what this will cost you in just parts, provided the truck is already cherry/perfect and you will be doing all the work yourself?
Have you looked at the truck market for buying and selling? If your truck is in great shape, it will fetch more $ now, likely than ever in the past and maybe the future.
Are you rebuilding the Cummins and the Alli, or just tossing junkyard drivetrain in it?
Have you considered everything else that will or may be needed? Cutting/fabbing core support, relocating hood latch, moving/fabbing trans cross member(s), adapting or purchasing new T case....just off the top of my head.
And if you get the whole drivetrain swapped, everything working, the least of your concerns is brakes and springs. On a cost and difficulty scale, the brakes and suspension is a 2 out of 10 compared to the swap, which is about a 9.9 out of 10.
Not trying to pee on your parade, but this doesn't sound like the making of a great tow rig, this sounds like the making of an expensive pain in the ....
Here's a quote from a guy who put a 12V into a GMT 400 burb, and he used the original trans and T-case.
"With the engine I think I am about $7000.
Between the flexplate, the trans adapter and the controller, $2000 of that was just to keep the 4L80E (ouch) but I didn't want to adapt a Dodge trans in it to the transfer case and then figure out the speed sensors, etc."
This is a stock used, basically "junkyard" 12V mated to the anemic (for a diesel) 4L80 trans.
If you are doing it "right", I'd guess about $15k minimum in parts and if you're successful in the end, you'll have a truck that is worth no more, really, than as it sits now.