I have fixed a couple of TF trucks that had the mounts installed as you mentioned. It will work for a while but the engine and trans will eventually flop around until something breaks.
The original automatic trucks in that era used a similar rear crossmember which bolted to the sides of the hydromatic bellhousing. If I remember right the mount kits came with instructions which had dimensions and locations. You have some flexibility about where to install the mounts when making a later model engine/trans conversion.
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