The car engine had a powerglide in it so I'm using everything off the truck engine.
The mounts on my 57 are not on that front plate. I used to have a 50 Business Coupe that was setup like you describe. I suppose I could re-locate the mounts forward on the cross-member and use that plate as you suggest.
I was also thinking of getting a V-8 conversion kit and using that. It would seem that it should work since the 6 cylinder mounts in the same place at the V-8 on the Belair.