Rear main could be leaking oil onto the clutch maybe? Or maybe the clutch linkage is binding somewhere and not letting it engage all the way. I know a lot of the parts on the clutch linkage use press-fit parts that come loose.
My dad has pretty much the same setup in his '81 F-150... 5.0, 3 on the tree, 9" rear (but with 3.20 or 3.50 gears... I forget which) and it will **** and git.![]()