Are you 100% sure that you have a transbrake valve body installed in your Powerglide? Many (not all) Powerglide transbrake valve bodies require the transbrake to be activated in order to have reverse gear. If you have reverse gear with no transbrake activation then there is always the possibility that your transmission is simply equipped with a normal full manual valve body.
If your transmission is indeed equipped with a transbrake valve body then you need to make sure that the brake valve and spring is installed. This is the valve that the solenoid pushes against when activated.
Do you know any details about how your Powerglide is built internally? A transbrake-equipped Powerglide should be equipped with a bare minimum of a quality aftermarket input shaft, billet high gear clutch hub, and 5 or more premium high gear frictions. A 1.76 gear set is preferable over a 1.82 as the carrier for the 1.76 gear set is significantly stronger, and will provide dramatically improved durability. As power levels increase more aftermarket upgraded internal components become necessary. A 1.76 'glide built as described will typically withstand 600 - 750 flywheel horsepower for a very long time.
As a side note, an original GM Powerglide band in good condition is the best band you can ever put in a high performance Powerglide. Aftermarket standard width Kevlar and high static bands are also great choices and will offer outstanding durability and performance, but an original GM band is still the best. When using a Kevlar band you only want to run the one with white lettering on the friction material. There are some generic Kevlar bands out there that feature the green lining, but with no lettering on them. These are inferior quality bands and will not typically last long. We have ran over 3,600 flywheel horsepower at 3,250 pounds on a stock GM Powerglide band without any issues. The "wide" bands that are out there in the aftermarket are a total waste of money... they offer zero additional holding ability on the drum.
