As I said in my post on another site, the 318 mounts you have should work. That is provided your 58 was an V8 car to begin with and the motor mount cans are for the Poly and not Big Block.

Savoy your mounts will bolt to the block but won't come close to fitting the frame mounts. Here is what the engine cans look like. there is a right and left, and it does matter if they are mounted to the front or rear of the engine mount lugs cast into the block.