The difference was rubber compound, not size/shape. The non-convertible mount fits and works just fine in convertibles. Sounds like something else is the matter with your setup; I'd guess your trans mount is badly sagged and/or you're working with incorrect motor mount brackets.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

The transmission mount is old stock US made and has very few miles on it, and all the brackets are original. Everything I've replaced over the years has been Mopar, and looks like original. The engine was never rebuilt and I just swapped a '79 in place of the '64. There is also no visible collision damage anywhere except at the driver's door.

I don't know what the problem is, but the poly mounts are shorter than both the OEM and the aftermarket parts and won't work in my car.

Did the 64 engine's oil pan & pick-up tube get swapped onto the 79 engine during the swap?
DD

Yes.

I also have a 64 vert.................does anyone have a parts number reference to check that a conv. lists the same part number?

best regards max

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1966 barracuda 360
1964 signet conv. 170 turbo
Bonneville or Bust 2012

The convertible doesn't list the same part number. It lists a different part number. Again, the difference is rubber compound, not dimensional. There's something tweaked on the OP's car.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

long as it works for a pass ..................life will be good

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1966 barracuda 360
1964 signet conv. 170 turbo
Bonneville or Bust 2012

If for whatever reason your engine is sitting too low, shim the mounts with plywood and call it good.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I've raced worse..............after 45 years...........if it just doesn't work out I have access to facilities to do a short run of ones that do...............so convertible people be happy ................we have resources.

take care, life comes at you very quickly

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1966 barracuda 360
1964 signet conv. 170 turbo
Bonneville or Bust 2012