8 3/4 to Dana 60

Why spend the time and effort to peel of .060" from the tubes?

Perches for 3.125 tubes are available from places like Calvert. Weight saving, meh, not going to be significant enough to notice. JMO having worked on this stuff over the years. Same as the Dana weighs more, car should be slower, nope, no change.

I agree with the preheat and using 7018 stick IIRC or you can try with MIG and std wire. Problem will be impurities in the cast section when you do it. Preheat pretty darn hot and try to slow cool it.