Normally the original bellhousing crossmember is removed when a late trans is installed. This facilitates the need for after market crossmembers which use the engine center mounts and the transmission rear mount. If you use the stock V-8 mounts and the trans rear mount there is too much distance in the center which is not supported by anything. I have fixed several cars & trucks that were done that way. On one of them the only thing holding the engine and the trans in the vehicle was the exhaust system as both front and rear mounts were broken from all the movement.
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