Chrysler changed from a flat rectangular mount to a round engine mount around '72-74 (depending on body type). The older flat mounts can/will break allowing for the engine to move all about the engine compartment (not a good thing).
The newer round mounts are made so if the rubber breaks, the engine can not go anywhere.
Also the newer mounts (the ones used for FMJ's) are almost (but not quite) universal – but Left sides are different from the Rights (but not sure why made that way).
The '72 and newer B-body had round mounts, '73 for A-body & '74 for C-body. The E-body (and pickups/vans) never did go to the round mounts for /6 or LA engines.
All /6 passenger car (not pickup or van) mounts, from '73ish to '84 (A/B/C/F/M/J bodies) are the same.
All big block mounts, from '73ish to '78 are the same (B or C body, but again, not trucks/vans).
All small block mounts, from '73ish to '89 are (almost) the same (A/B/C/R/F/M/J bodies) but trucks/vans and E-bodies. With that said, all Right side (round) engine mounts are the same. The Left side mounts are different between the 273/318 and 340/360 engines – by about ¼ inch (just enough to be noticeably different).
There is absolutely no way to put a 340/360 Left mount on a 318. A Left 318 mount can be put on a 340/30 if you install “about” ¼ inch spacer between the mount and block ear – but my experience is using the spacer causes that bolt to loosen over time. My recommendation is to find a 340/360 Left mount if installing a 340/360 engine into a car.
From the factory, there are minor differences between the hardness of the engine mount rubber. The police/taxi/station wagon's received harder (rubber) mounts than other cars did – but I don't think most people would notice any difference between them. I also don't think one hardness lasts any longer, either.
Aftermarket mounts only use one (rubber) hardness.
The K-frames are different (all bodies) between the flat (older) and round (newer) engine mounts.
All K-frames are different between /6, small block or big block for each of the body types (for round mounts). A person can convert any (of same body) K-frame to different engine type by cutting/welding (and necessary fabrication) – but it would be easier to just get the correct type K-frame to begin with.
The only two different K-frames used for FMJ's were one for /6 and other for small block. There are some minor differences between the years – but not enough to cause any issues.
I can't wait for the Schumacher mounts to get back into production and even had a discussion about that with a friend last night. By far, the easiest route is to use a sent of conversion mounts – for a K-frame swap is a bit more involved.
One plus side to replacing the K-frame would be is
if you need to overhaul the suspension, then it is easier to do it on the bench then than under the car.
BudW