One wire lock-up, 4 spd automatic w overdrive into A-body.

1/4" steel should be plenty!! If you pull on the tail of the transmission without first loosening the motor mount bolts you'll be putting tremendous stress on the mounts and K frame. I would suggest loosening everything up front then get the tail where you want it, then retighten the front. If the tail springs back (out of center) it means that there will be some lateral stress on the mounts when you're driving down the road. the transmission should sit in the correct position naturally without a lot of pulling after you get everything positioned properly. The distance from the motor mounts to the tail of the transmission is probably 4-5 feet. That's gives you tremendous leverage on the tail. 30 lbs of lateral pull on the tail probably put 300 lbs of stress on the mount area.
Why does that 1/4" plate cut out look so familiar???


treblig