Classic/Antique Car Repair: powerglide transmission faliure, flare nut, tail shaft

Question
we have fitted a second hand ( first mistake ) alloy powerglide to our '55 chevrolet after the demise of the cast iron unit ( car has tow bar, second mistake ).No mods to the box that we are aware of.The oil level when engine is running is showing full, I can crack a flare nut on the trans cooler a get oil pressure but have no drive or reverse gears. The other tell tale is a loud ratatattat which eventually grinds to a halt upon selecting park while the engine is running, I'm assuming this is the park detent.Any thoughts?

Answer
The parking pawl is connected to the output shaft inside the transmission. because you have a ratchetting noise when you try to put the rig in park tells me that there is motion to the very final part of the transmission. This means that one of the following is a possibity:
1. the output shaft is broken in the tail shaft of the transmission.
2. the drive shaft is not engagining with the out put shaft spline.
3. check to see if the drive shaft is turning with the engine running and in gear. If it is the problem is in the rear end.