Plugs For (thermactor) Exhaust On A 66 V8

Need to replace the (Thermactor) exhaust plugs on my 66 V8 with something like C6AZ-6052-A. Is there an alternative, like a regular 1/2-20 bolt? I assume a grade 5 or 8 steel bolt should work, about 2" long, but don't know for sure, is it exactly 1/2-20 thread? Any suggestions are much appreciated (or pointers where to find the old part#).

Thanks!