To heat the offenhauser you've got two choices, water and exhaust.
Both involve bolting a flat plate with two holes threaded into it over the 3"x4" divided opening in the center bottom of the offy manifold. The divided box leads to passages inside the intake that go to the plugs. If you run heated gas or water in one side of the divided box it'll come out of the other side after having circulated through the passages in the bottom of the manifold. Heating the manifold, as many other posts have noted, is critical to atomization of fuel during the colder months.
If you use water, tee off the thermostat housing to heater outlet and the inlet to the water pump and run both lines to the offy.
If you use exhuast gas, use the two holes in the fentons. If not, put some threaded plugs in there-stainless or brass are better 'cause they won't bond with the cast iron.
I went with water and my offy leaked til I welded up the plugs. Now it still leaks 'cause I ground it a little to much off the bottom of the offy and I think I made it too thin in the water jacket area. (that's why I'd buy yours if you decide you don't want it).
To grind the hold down tabs I just bought a 4" grinder from Harbor Freight and it came with a cut off wheel and a grinding wheel for metal. I used the grinding wheel and it worked fine. The trick to grinding the fentons and offys is to start by lying both of them on their mating surfaces on a bench and start with the mount flanges. Then put it up to the head, tighten it down (no gasket) and mark the outer tube mount points and then grind them. It'll take several rounds of this to get that part just right, and it'll make a huge mess of metal particles and dust from the grinding wheel so don't forget to wear a respirator/mask, eye protection and gloves.
If the plugs on yours are heli-arced, the plugs are aluminum (you can't weld steel to aluminum). Some people get lucky and their plugs never leak.
As far as removing the studs in your head and replacing them, another thing you could do is put threaded studs in. That would give you the benefit of having something to rest the manifold on while you line up the compression bars and you'd have something you could tighten down on the ends!
I havn't had any problems with my head pipe leaking and I used the flanges and gaskets that came with the fentons.
Soon you'll come to the question of what to do with your exhaust. I suggest y-ing into one pipe with a y that comes together right at the crossmember the engine mounts are on and then running 2 1/2" pipe from there to a single muffler. Of course theres lots of debate on that topic and opinions and theories vary.
Good luck!
