It looks like the driveshaft on my 68 Chevelle is too long. The 10-bolt is tipped slightly forward and the slip yoke at the front is pushed tightly into the powerglide transmission housing.

Any ideas on what the driveshaft length should be?

Your driveshaft should have one inch of slip yoke sticking out of the tailshaft.

not sure but long can be cut and short means trouble

I believe the correct length for a Powerglide trans driveshaft is 56 1/2" from center to center. The only one I have is in my 69. If I remember, I'll see if I can get a measurement off it.

Are you checking it's fit at ride height? The driveshaft is pushed forward and trans yoke pushed more into trans when the differential is hanging down unsupported and no car weight on the rear suspension. With the diff supported by jackstands and car weight on the suspension (at normal ride height), or the car on the ground not supported, the driveshaft will be more to the rear and yoke out of the trans more.