I think there may be more than 3 or 4 different brackets for these GM HEI starters, as there are at least 4 different starters for these trucks, with only slightly different lengths of starter housings. I also think that you should follow your manufacturers recommendations about adding a time delay that gives the starter a 1/2 second of running before any spark gets to the motor.
This problem has to do with how much compression these motors hold, and how hot a spark they get when the key turns off for a split second between "on" and "starter crank", which causes a spark as the coil field collapses, that ignites what ever fuel and air are still in the cylinder, causing a 'rock back', turning the engine backwards just as the starter engages.
I also always want to make sure the key is of the type that once it is release from the "Starter crank" position, it has to be turned all the way off before it can be turned to "starter crank" for a second time.