40 + year old engine mounts, are you kidding.
Safety first. Try a set from Year One. Good factory replacements.
By the way I like your build thread. My first car was a 1968 GTO. I now have a 1969 GTO Judge resto-mod that I had since the age of 16. Now over 40 years.
Nice attention to detail. I spent two years detailing my parts. This is what takes the car to the next level. Keep up the good work.