727 transmission problem
For starters you want to check the valve rocker arms for loosness. Most of the pushrods will spin with little effort, but there will be a couple of cylinders where the valves are opening and those push rods should be tight as the lifter is riding up on the cam lobe to open the valve.
As Pictured here on cylinder #4. On the intake valve opening.
Loop the trans cooler lines together with some rubber hose at the tranny. Then start it up, check the fluid level in neutral after shifting it through all the gears.
Want to make sure the trans has the proper fluid level for starters, don't over fill it either.