I've never poured engine mounts but I have poured a few smaller items, including some vibration reducers for a prototype shaker mechanism and some tubing mounts. I used shore 70 and 40 polyurethanes from www.smoothon.com
. We had a vacuum pot handy so I used that for degassing, without it there were lots of small bubbles. I finished with an overnight bake at 150F, which really helped toughen the rubber. If you are going to do a lot then it would be pretty easy to rig up a pressure chamber instead of vacuum. The simplest way is to just buy a HarborFreight paint pressure pot: http://www.harborfreight.com/2-1-2-half ... 66839.html
. It's rated at 60 psi, and smoothon recommends 100 psi, so it may not be quite enough. If you do vacuum degassing make sure your mixing pot is at least three times the volume you are mixing or it will boil over. Since we had a full cnc machine shop I just made my molds from aluminum. Didn't polish them, just left the surface as milled, and got pretty nice parts. Oh, I used their aerosol mold release. Anyway, if you have any questions either post them up here or email me.