I rescently started down the same path with my '71. Manifold was leaking at the head & I couldn't stand the old farm truck sound that resulted. I used ford brand rust penatrant (the best made in my experience) and soaked 'em good for a couple days in a row. I managed to get every bolt out except one! My best option was a metric 6pt impact socket (fit very tightly) and a gentle hand on a really long breaker bar. I would not waste my time heating unless you have oxy/acy setup as most of the bolts thread into a meaty part of the head. Once I'd broken the head off of the frozen bolt, the manifold fell out of the way. I tried to reweld the head of the bolt back on the stub three times & broke it off three more times. Each time i broke the bolt off it was that much closer to the head. Eventually it was time to take the head off and drill / helicoil. Once I had the head off I realized how much attention the valve train needed... What started out as a quick e manifold gasket replacement has turned into: Valve job (new valves, ground seats, new guides, retainers & seals) Resurface Head Resurface e manifold. gasket had blown due to a rusty pitted surface. New gaskets Repaint valve cover... & Countless new fasteners Should be back to healthy this weekend. Moral of the story, be very careful with e manifold bolts!
I must have been very lucky. When I topook mine apart, all the bolts just came out with no problems at all. Good luck to you
