I think you are on the right track. There is too much distance between the front mounts on an early engine and the rear trans mount to support both properly. It is a common mistake that people make when converting to an auto trans. I have seen examples where the only thing that was holding everything in place was a rigid exhaust system. I have used the Danchuk bellhousing mounts which were designed for that specific purpose. The Danchuk mounts are a little pricy and you would have to modify them anyway so I would suggest you fabricate some bellhousing to frame mounts to suit your application. It would probably be a good idea to use a rubber mount pad on each side of the frame so there is some flexibility.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine