When I rebuilt a '90 Mustang GT 5.0L v8, the guys at the local mustang shop recommended the permatex white "thread sealant" that you can buy at most auto parts stores.
I'm assuming you're talking about head bolts that are bolted into engine blocks where the threaded holes in the engine block goes straight through (is not sealed/blind). Late ford small block 5.0's are like this and you have to use thread sealant on the head bolts because they pierce the water jacket.