[blykins]

How's the driveshaft?

Does the driveshaft turn when the tranny's in gear?

If the driveshaft is intact and doesn't turn when the trans is in gear and the clutch is out, then yeah, it's your tranny.

Yes, you can upgrade mainshafts. When I rebuilt my HEH-G, I upgraded to a new 28 spline mainshaft, a full road race rebuild kit, and some other goodies.

You'll have to change tailhousing bushings.