6inarow, Tom, et al.,
Before just drilling the front crossmember to accept the 1951 and earlier front mount, please read pages 6-18 through 6-20 of the
1949-53 Chevrolet Shop Manual .
The rear side mounts that react the engine torque must also be installed between the frame and clutch housing; if not, the car will not be pleasant to drive. I learned this the hard way - twice- in 1958 and again in 1960. In 1958 I installed a 1954 261 in my 1953 210. The bosses for the sidemounts were not drilled and tapped for attaching the brackets, so, not wanting to take the time to drill and tap the six 3/8-16 holes, I drilled the two holes in the front crossmember to accept the 1951 front mount. The front engine plate was already drilled. The engine did not fall out, but it was almost as bad. The torque reaction would first cause the engine to rotate so much that it was hard to shift. Then, after a couple of weeks, the front motor mount separated, and the car was barely drivable. It was then time to install a new rear main seal, a hotter cam and some other goodies, so I pulled the engine, acquired a good drill motor, and drilled and tapped the bosses.
Two years later I installed another 261 in a 1951 Chevy, which does not have the frame towers for the side mounts. Thus I decided to stay with the 1951 mounts. However, I forgot to reinstall the sidemounts. As soon as I started to drive it, I realized what a dumb thing that was. With the rear sidemounts back in, the car was drivable, but not as smooth and well-balanced as was the 53 with the good sidemounts.
The engine mount experts that I work with tell me that the engine mounts should be aligned with the "principal axis of inertia" of the engine, which generally passes through the center of gravity of the engine assembly. The 1952-54 mounts are much closer to meeting this goal than is the earlier mounting system.
If you have the time and inclination, using the 1952-54 sidemounts should provide for a "better" installation. However, the 1951 mounts will work, as long as the 1949-51 rear side mounts (or a home-made equivalent) are used to react the torque.
