A couple of things to consider, if you're keeping the stock 7.5 rear end, then upgrading u-joints is not necessary due to the rear end will blow up before the driveshaft will. Don't take that comment the wrong way though, the 7.5 will live for a good while as long you have 'normal-ish' street tires meaning don't launch this regularly on sticky tires. 2nd, to measure for a driveshaft, measure from the center of ujoint cap to center of ujoint cap with the car sitting on the wheels with normal weight on the wheels. Meaning don't do this until you have the 455 and trans installed in the car with all suspension upgrades completed. From there stick the trans yoke in as far as it will go (bottoms out), then measure from yoke to yoke and deduct 1.5".
This process is somewhat of a PITA id you don't have a drive on lift, but it can be done. X2 with FleetPride, but Albany is a large area, ask at one of the speed shops or machine shops. Lastly, don't reuse your old tube - you can re-use the yokes if they are still in spec.