If you break the heads it's a major pain to remoove properly BTDT. The screw is in the same kind of situation as the ones that are holding the rear brake disc or drum on many cars. The remedy is often the use of an impact screwdriver. It's an screwdriver that you make turn by hitting it with a hammer. WD-40 isn't good enough, go to Canadian tire and get some Liquid wrench (yellow container) to free rusted bolts, it really works well but can't do miracles if it's your case. If you want to give a try at the screwdriver CT has them too, just practice on other screws before so you can figure out how it works properly.
1973 porsche 914 2.0