Yeah the electric impact wrench I have is a cheapie but it works great for what I need it for. MY application is a little different so I am not sure what size the bolts are and I will have to ask. Mine is a Pontiac OHC motor and the manifolds are bolted together and then bolted to the head. The Main bolts that hold everything to the heads are pretty big so they might take the impact wrench. I am not sure a socket/extension/wrench will fit to get them out but I'll try it.
Here's a pic of what I am working on:
I'm pretty familiar with that style engine...
Head over to www.inliners.org
. Great site for inline 6 engines.
Mine: Offy intake, Langdon headers, 2.5" single exhaust to a Flowmaster 44, Holley 8007 390 CFM carb, HEI distributor, and electric fan conversion.
...and still DON'T USE AN IMPACT WRENCH ON YOUR MANIFOLD BRIDGE STUDS AND BOLTS.
(had to modify that a bit for you...)
To get it off you have bridges studs and nuts. The bridges are between the intake and exhaust manifold. There's a single stud between and a bridge. You might not be able to get a ratchet wrench in there. I believe the nuts are all 9/16". There are no nuts/studs on the ends. Only between the manifolds.
Step 1) Soak the heck out of it for a week in PB.
Step 2) Remove carb.
Step 3) Unbolt exhaust from exhaust manifold
Step 4) Remove power steering and bracket.
Step 5) Remove all vacuum lines.
Step 6) Break studs/nuts loose with box end of wrench.
Be sure to have a really good gasket on hand. Also I recommend copper RTV. Sealing the manifolds to the head is a PITA. I wouldn't take the intake apart from the exhaust unless you have a darn good reason to do so. I believe there are three bolts to do it. Two are visible. One is hidden away under a heat shield on the exhaust manifold (if you still have yours).
edit: Why are you removing them? I might have other fixes for you if I know a bit more.
If you want more power there are several options. Inliners really is the best way to find out about what's possible.