I've replaced small blocks with big blocks in '63, '65 & '74 Impalas (& a '71 Holden Monaro) & the only mount "modification" that I had to do on any of them was on the '63 - it needed a couple of washers under the frame mounts for pan clearance. Ignorance is bliss.
GM didn't bring any cars into Australia with big-blocks, so I've never seen the factory set-up.
I'm really curious, what is the difference in the frame mounts?
Are they different dimensions or perhaps beefier construction?
If the dimensions are different, I'm wondering how I got away with it.
If they're same dimensions but stronger made - I'd better beef mine up before I put the motor back in the '65.
i would figure they would be the same i heard there were only 2 types of mounts 1 being for engines of any kind below 250 hp and the other for engines 300 hp and above my impala has the 350 300 horsepower engine in it but i wanted to get second opinions before i went to try and do it only to have to get other mounts
The frame mounts are different only on the pony car (Camaro) and compacts (Nova) that had a big block installed. The Chevelle and the full size Impala used the same frame stands for a small block as they did for the big block because they have a full frame and a bigger engine compartment to swallow the engine than the smaller cars.
There are actually four motor motor mounts made by Chevrolet. There was always a difference in motor mounts based upon the horsepower. With engines making 300 horsepower or more having interlocking motor mounts, while the base engines and six cylinders used two plates of steel glued together by molten rubber.
In addition any car made from 1969 to 1972 has a different motor mount than the cars made from 1958 to 1968 because of a law suit brought by Ralph Nader in reference to the base engine motor mounts breaking. There are still two different 1969-'72 motor mounts with high performance motors getting a stronger part than the weaker motors with 300 horse being the dividing line (except for the wimpy Z/28 302 that only made 290 factory rated horsed power though the NHRA made it closer to 320 in stock trim; SCCA had cars with the little 302 pushing close to 500 horsepower).
The broken motor mount allowed the engine to roll over in the engine compartment and pull the throttle open as it did so because they used a solid steel rod the attach the pedal to the carb. After 1968 all motor mounts were interlocking in design differing in their size from the earlier motor mounts so that an old motor mount wouldn't fit on the frame stand.