As stated a sound structure is the key factor. Look for rust in the rear frame rails and shock towers. The rockers are another critical area to pay attention to. Also the metal around where the sub-frame attaches.
All the skin and structural metal are repoed for FG ‘birds and Camaros now. So even if there is rust thru it’s not a problem that can’t be corrected.
The simplest answer to the heat issue is bigger CI = more calories burned = more heat. There are solutions as you'll find out. It's a 'hot' topic around here (pun intended).
If you do wind up getting this car you’ll make Tom #3 with a ’69 convertible that I know of. Welcome aboard!