Damaged trans?
I have to tell you, I would NOT take the chance. Back in 68, the guy who would become the original owner of my 70 sixpack RR had a 65 Chevelle with a 350/ 4 speed from a wrecked Camaroo, and we'd put a 350HP cam in the thing. It ran pretty damn good.
We were going to Navy schools at Treasure Island, and he pulled a "big" 1-2 shift on the on ramp from T.I. to San Fran. Ker -- freeckin' -- blaooey!!
We got 'er back to the base, bought a junkyard (10 bolt) axle, and put it together. IT WOULD NOT MOVE!!!
The shock of the rear axle breakage had SNAPPED THE input shaft of the Muncie IN HALF!!!
See the groove in the middle of the splines? I have no idea why it's there, but that is where it broke