Welcome to the Team Bill!
When the SBC was introduced back in 1955 it had a 265 displacement and made 162 horsepower. In 1958 the 265 was just a memory replaced by the 283. It also had a new motor mounting system. Instead of setting the motor and transmission down into the frame bolted to four legs like a stationary engine the way it was in 1955 through 1957 they had a new tricycle mounting system. Two mounts on the block and one supporting the transmission. The mounts were still using gravity to hold the engine in place as they where pieces of steel stuck into hot liquid rubber in a mold to make a mount. The 283 was already making 190 horsepower or 28 more horses than a 265 but the engineers were satisfied that the mounts would hold the power with a 50% safety factor which was the engineering norm back then.
1962 the 327 is introduced it is now making 275 horsepower. In other words the base engine was already within 10 horsepower of the upper design limit and that was just for the base engine as there already existed a 350 horse variant that was installed in the Corvette with the old style four corner mounting system that would easily have broken the motor mounts in a Chevy Passenger car (in sixty two there were four Chevy Models; the Corvette, Full Size, Corvair economy car, and the super economy Chevy II that was just introduced with a four cylinder motor). However starting also in 1958 there was a big block variant that was making far more horsepower and torque. It had it's own motor mounts that had different dimensions and they had an interlocking mechanism to prevent breaking the mount. Because of this interlocking mechanism they were physically thicker or taller depending upon your point of view.
The 409 really aggravated the motor mount problem as did the introduction of the 360 horse 327 into the new intermediate size car in 1964. Motor mounts started failing causing the motor to roll to the passenger side pulling the throttle open because it used a steel rod instead of a pull cable before 1969. As little old ladies started mowing down tea party attendees at lawn and garden meetings the complaints started pouring in . Starting with a law suit filed by Ralph Nader who had just lost his last law suit against Chevrolet claiming Corvairs were unsafe. This time he won. In 1968 all 1958 through 1968 V8 equipped cars were recalled in the first in history safety recall mandated by the federal courts.
As a result of that law suit safety straps were installed in all recalled vehicles to prevent the motor moving if the motor mount broke (which by then most had). Chevy also changed the shape and size of the motor mount to include an interlocking feature on all motors rated over 300 horsepower. These motor mounts were called big block mounts but they were used on the little 302 Z/28 as well as big blocks. The sixes and base engines (even the 230 horse 350 SBC got a newly designed vulcanized non-interlocking mount.
That means that there are FOUR different motor mounts used on all V8 engines before 1973 when the clam shell motor mount used today was introduced. Problem is the corner car parts store has only one motor mount listed in their computer for any Chevy V8 that actually only fits the base SBC engine low horse 1969-'72 307, 350 motors. These mounts won't fit older cars but the catalog says it does. They won't fit high horse cars but the catalog says it does. And since the kid (of any gender or ethnicity behind the counter wasn't born or has ever seen a carburetor powered American V8 he (or she) has to rely upon what the computer says).
That is why you get the wrong motor mount.
Been there, done that, got the T-shirt