I'm working on a 1972 455 jet boat. I have to remove the 40+ year old rusty exhaust manifold studs. I DO NOT want to break any or the heads. Any experienced tips on best practices? TIA.

 

Heat, and lots of it. My personal favorite technique is to heat the stud/nut/bolt/whatever hot, and then soak it with a good penetrating oil. Do this several times, the idea is to try and get the oil to wick into the threads. After your confident enough oil has made its way into the threads (or your tired of the smell of burnt oil) get the metal glowing red, get a good grip with vise grips, and cross your fingers. If your patient, the bolt will come out. If not, grind the broken bolt flat with the head, drill the exact center of the broken bolt with a small drill, and keep going up in drill size. Do it right, and you will be able to pick out the remaining threads of the broken bolt, leaving the threads in the head relatively intact.

 

Weld a nut onto the stud, heat the head area, then use an impact thread..

 

I have heard of using candle wax to wick into the threads after heating.