Originally Posted by mach4

On my 617->107 engine swap, my clearances were a bit tighter than stock so I filled the mounts with urethane to prevent collapse. With the right urethane the increase in vibration will be undetectable. (I've now got 70k miles on them with no collapse) Now those are the earlier style mounts that have an exposed cavity that the urethane can just be poured in.

You don't state the car in your profile, but you've obviously got liquid-filled ones. The filling technique will be a bit more tricky, but I believe still doable and that's the tack I would be taking.

If you look at the cross-section of the later mount design you can see two chambers.



If you were to drill a small hole in each chamber, allow the fluid to drain out and refill with urethane, you'd have a mount that would likely outlast the car. In this application, the stiffest urethane you'd want to use would be Shore A 30 and I believe that a much less stiff one would be acceptable and likely even better. Because it's unknown whether there would be an adverse reaction between the original fluid and the urethane, I'd flush the old fluid out thoroughly with a suitable solvent.

Here's a durometer chart that shows the various options.