4 liter rxhaust manifolds.

Any experience removing exhaust manifolds on 4.0 OHV Ford Ranger? They have 10mm heads. I "gently " tried on this afternoon - appeared VERY TIGHT. Fo I heat the heads, soak with penetrant, use an impact, or combination of any 2 or 3? What are the chances of breaking the bolts since the reason I need to remove them is to replace broken exhaust studs. What size are the manifold bolts?? All the dealer could tell me is they are about $18 each and there are 12 of them. Also what size are the exhaust studa. They were less than 6mm thick when I attempted to remove them.