Power glide-no reverse

[NHRA1926]

Car weighs 3700 with 200 lbs driver, estimated 650 horse, 10.30's @ 128 mph, 1.35-1.38 60ft with 2.74 1st gear with turbo 400. I was always told a 400 is stronger than a glide, but a lot of local racers have switched to power glides. They talked me into trying one, so I borrowed one from a friend that came out of a super stocker. Brought it back to the local tranny guy today. He installed a plate behind the tail shaft housing to cover the cresent channel. He Said that should fix the reverse issue. Brought it home, installed it back in the car. Still no reverse, and trans brake does not hold when activated in 1st gear.