I put a 455 Olds / TH350 in my '81 Firebird and had a 403 Olds before that. The 403 Motor Mounts worked fine. You can buy/order them at most Parts Stores. My Mechanic likes the GM ones. I've only been finished about three months and am very happy with it's performance. Best 1/4 mile is 11.86 sec @ 112 mph. I've only had limited run time and expect much more.
Have fun with your toy.
455olds@74