For Pontiac engine you'll need Pontiac mounts to attach it to the subframe. The chebby's won't work. Trans bellhousing is also different, and possible trans xmember changout may be needed as well to get the angle right. And likely driveshaft length will be wrong after you get it set in place. A lot will depend on whether or not the original Chevy conversion was done correctly in the first place, and whether you're pulled out a SBC or a BBC, and possibly the age of that motor you're pulling.