For what it's worth, I had a buddy with a '71 duster 20 years ago. When he got it...It had the top of the cross member cut out exactly like you've done to make yourT400 fit. Someone had chopped his out to put a 3 speed manual in the car, and the cross member was in the way of the shift rods if I remember right. This was a lowly mid 13 second car... nothing crazy.

Anyway... We converted it back to a 727 almost right after he bought it.

The odd thing was that it would break flex plates and trans mounts on a regular basis. After about 4 or 5 of them (even aftermarket SFI ones), we suspected maybe the missing frame cross member section had something to do with it.

My dad cut a piece of cross member out of a donor car and welded it back in for us. Never broke another flex plate again!!!!!

So just my 2 cents from my one and only experience.... with that crossmember piece missing it was a disaster.