Rob,
You said
I run a 3.21 to 1 (66 Toronado Planetary Gear FD) in Double Trouble. I don't know what the rpm's are at 60 but I do know that at 70
the engine is turning around 2400 rpm. The engine absolutely LOVES that rpm and will do it all day long.
I would like to know what, if any, modifications to other drive line components are required to use the 66 final drive in a GMC? I ask because I have a fair number of these planetary final drives in my collection of Toronados. I know the inboard universal joint is unique to a 66 but hoping the stub axle splines of the GMC will fit the 66 FD.