Scott 4 Chevy,
Do you have and are you planning on using the original 1951 bell housing???
If you are intending to use the 1957 bell housing that came with your 1957 235 and SM420, the mounts on that bell housing will not be a bolt in to the 1951 frame.
According to this page
Parts Group 0.683 Housing, Clutch and Flywheel in the 1929-1957 Chevrolet Master Parts and Accessories Catalog, the bell housing for 1948-1954 do not interchange with 1955-1957. The mounting flanges and mounts themselves are different.
Others will most likely chime in here with additional information and suggestions.
Hope this is of some help.
