Personally, I don't see any advantage to a poly mount over stock rubber. With the exception of the FED, I've always run rubber. I suppose if you want your mount to be colorful or shiney, use poly. If you just want to have a mount on your tranny and have better things to spend the $ on, use rubber. Just get a new, one not some boneyard piece!
As far a motor plates go, yes just a front plate will WORK. If you have room, I'd put in a midplate and a lateral limiter. Especially with a bigblock. That front plate will get bent eventually.....
JMO,
Johnny