There was no specific mount used for 69 FB 400's.
Same as all 67-69 FB V8's. Also mtg brkts.
Had pontiac o/pan's from 65-78 in 2-68 FB's. None touched.
There are 4(2-upr/2-lwr ea side)steel mtg brkts correct?
Should only have about an 1-1.5" between back of engine and firewall.
My guess its the trans x-member and/or trans mnt push'n the rear of engine/trans up.
Be'n original x-mbr(not for bbc/sbc), i'd try removing mount and see how it is just on support.
The engine will rock back into firewall.