I was wondering that in the previous posts. Heat being generated by the trans is energy that could be put into motion rather than lost as heat in the transmission.I'd run a full radiator, but I'd be scrounging for one from a junkyard and expect to pay $200-300.
Those AT545 is reliable and cheap, but it also might well be the very worst transmission you can own in terms of heat production and efficiency. A new rear-end will just make it slip more and make even more heat. My Allisons are all behind 6.6/7.3 I6 motors, but swapping to an MT-series auto (643) should be possible. If so it would reduce your heat load while increasing felt power and speed. I have two trucks that are very similar in size/weight and the main difference is AT vs. MT series transmission and the MT truck is quite a bit faster and gets much better mileage. Of course an MD transmission would give you O.D. as well, so that would save you the rearend swap.
I think this is with a DT motor, so your process will differ: https://tatumskoolie.wordpress.com/2016/05/05/swapping-allison-at545-to-allison-mt643/