Drive shaft length in a T bucket is based on several things - chiefly the engine and transmission length and placement in the chassis, the key items. The one I built several years ago was based on a Total Performance chassis with a Ford 351W/C4 transmission. The engine was ~4" ahead of where they suggested and with that placement, had a 9" long drive shaft centerline to centerline of the u-joint bearing cups. This could have caused some problems with u-joint wear, but it was anticipated, and for <$25/year changed them out, JIC (they were never bad)
Dave W