question on mounting power disk brake booster on 65 cuda

so i got a kit to convert the front of my 65 cuda to power disk brakes. the one thing i am confused it no matter what i tried the booster and bracket for the shaft that pushes in was hitting the firewall. i called pirate jack where i got the kit and explain. He said oh you have to cut the rod all the way back to the boot to get the clearance. Really? little nervous doing that cause if i mess it up don't know if i can replace that. suggestions? comments?