I'm working on a 57 3100 and have a similar problem. I'm using the CPP 5559MCA bracket and a 69 C10 drum/drum M/C (#1329). Mine is a non-boosted setup. It seems like the pushrod that CPP supplied isn't long enough. I haven't done a trial fit in the truck yet, but it looks like about 4" of slack, same as the OP.
regards,
Leon
