Replaced the polyurethane motor mounts w/stock rubber type, couldn't get the frame brackets to bolt up right (too wide) ,plus I prefer the isolation of vibration of the rubber mounts, I bought the 69 Camaro tall/narrow type of frame mounts from Year One, almost ready to reinstall the engine. here's some good Chevy motor mount info:
http://www.nova-ss.com/MOTORMOUNTS.htm