A converter that has a piece broken off can plug the exhaust if the broken pieces turn the wrong way. The converter piece has small holes the exhaust passes through. When it breaks, it can roll and block off the exhaust flow through part or all of the holes in the the surface. Rattling inside the converter means those pieces are flopping around at are blocking at least part of the exhaust flow some of the time. Your exhaust flow can be part of your problem.

A partially plugged radiator could also be part of your problem.

Have you cleaned between the AC condenser and the front of the radiator? I've seen an amazing amount of stuff accumulate between the radiator and the condenser. That will also cause over heating at an idle.

If you are replacing the freeze plugs, there are 2 on a small block between the motor and trans. I would replace both of those as well.

Shelby probably didn't have the front sheet metal on the truck when they installed the motor. Gene