I believe that my aluminum driveshaft tube is 3.5" diameter. It clears the tunnel, parking brake linkage and exhaust fine normally, except for one time shortly after installing the rear and while still on the 6-cyl leaf springs, the rear end wrapped up and bounced the driveshaft off of the tunnel. With the correct leaf springs (Maijer 165#/in), I've never had a problem.
I did, however, discover just before heading to Grattan raceway last weekend, that the aluminum end-yoke had egg-shaped, leaving the u-joint's steel caps loose in the aluminum yoke where they're supposed to be a press fit. Not sure if that was a delayed result of the aforementioned driveshaft / floor collision or if it's the result of a simple overabundance of torque. (426ft-lbs, 445hp at the rear wheels).
By the way, I have no idea about an "Aerostar aluminum" shaft - not even sure what that means...
- Andy