Just curious, what's broken on your current transmission?
People can sometimes have fussy memories. I would at least pull the side cover of the replacement transmission and check it out. Take some pictures of the gears and synchros. You don't want to waste labor installing a bad transmission especially one that's attached to a torque tube drive shaft.
The side cover has a gasket. The real headache comes with replacing the seals on the torque tube which is logical to replace at this point. The seal kits are readily available at your FLAPS or stores on-line. There is a procedure to safely remove and install them. Take some time to read about it here on past Stovebolt postings. The procedure is in the shop manual.
http://chevy.oldcarmanualproject.com/shop/1948_51truck/index.htm The torque tube is directly connected to the rear axle which may have tension in the leaf springs. If you disconnect the u-joint inside the ball, the driveshaft can crack your skull. If you already knew this then you wouldn't be asking about the gaskets. Ask questions and be safe.
