The 3500HD axle has a Y type draglink and tierod, in my opinion not as easily adapted to these old trucks. The 5/10 lug I-beam axle from a P30 chassis has draglink and tierod like the old truck, easier to adapt, and in all other aspects is like the 3500HD axle.
The Dana 80 and P30 or 3500HD axle combo are good for a vehicle GVW of about 14,000 lb judging by the rating of the vehicles they came stock in.
A Dana 80 can be had with 3.31 axle ratio, or gears can be bought for it. The carrier split is 4.10 and lower numbers use one carrier, all higher numbers use a different carrier. The split on a Dana 70HD is 4.56.
