My 1971 455 mounts were alittle different than the 1969 400 mounts were. I drilled a few different holes for the mounts. I'm glad that I did. I was able to put an electronic distributor in it so it wouldn't hit the firewall.
That's the whole deal about keeping it pontiac to make it worth the cost of money you are going to be sticking in it. It's ok to stick money into the car, but not the motor because of it being worth more with the pontiac motor.
Eventially you can sell the other 455.
I'm doing a 1937 pontiac and did find a 67 catalina 400 motor for it. Took awhile, but will be well worth it when done.
I hope you get it because they are getting harder to find.