If you rev your motor and the motor moves excessively one or both of your motor mounts is broken. Visual inspection is the best method for determining the condition of your mounts. Jack up the car and put it on jack stands. Get underneath and look at each of the mounts. The rubber should not be cracked or missing pieces. Push on the motor with your hand and make sure the rubber is not broken and that both sides ( motor and the part of the chasis where it mounts ) are firmly attached.
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This you could get away with for awhile without further damage. Motor mounts have a built in safety device to keep the engine in place when the rubber mount breaks.
you take the mounts of and put new ones on dah
Check motor mounts
There is no fixing a broken motor mount. Replace them with a new ones.
the motor mounts should be the same, as the 318 is the same block as the 340, the only thing I can think of is the oil pan was different on the 340.