Don't bother trying to use the original mounts with the V-8 and 700R4. It will not support the engine and trans correctly as there is too much distance between the front of the engine and the rear of the trans. There are numerous aftermarket suppliers that sell tubular cross members for this purpose. One mounts to the center of the engine the other on the rear of the trans. You will likely have to remove the existing rear cross member.
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