Join Date: Sep 1998
Location: Mustang, OK USA
ALL of my experiences with, or friends experiences with replacing a SB with a BB have been to remove the SB, bolt the engine mounts to the BB and install the BB. Period. I have never seen or experienced thiis to be a problem, although for other people it seems to create a problem.
NOW, AS I HAVE SAID BEFORE HERE, THE CHEVROLET PARTS BOOK SHOWS BOTH THE FRAME BRACKETS AND THE ENGINE MOUNTS TO BE THE SAME PART NUMBERS FOR BOTH SMALL BLOCKS AND BIG BLOCKS in 68-72 CHEVELLES!!!!!!!!!!
Therefore, if you want to replace your 307/350 with a 396/502, there should not be a problem. There never has been for me.
There is a possible exception to this. There may be some Chinese made engine mounts that are less$$$$$$$ than US made engine mounts that are just a little too narrow to slip down over the frame brackets. whenever you buy new engine mounts, test fit them on the frame brackets first. If they fit, then everything should be fine. Otherwise, take them back and buy a better quality US made mounts. Other than that, there should be no difference in the dimensional location of the motor mount holes where the long bolt goes through the mounts/brackets.