Im getting horrible exhaust noise from under the hood so I was looking to switch from the stock exhaust manifolds to headers, but now im considering keeping the stock manifolds and just putting new gaskets on and hoping that will fix the issue. I will end up putting on new pipes and mufflers as well because the stock ones are rusted pretty bad. I cannot seem to find aftermarket headers specifically for my vehicle. I find tons of headers for the 352, but they say they are meant for the F-series and im pretty sure those wont fit between my rails.
You are correct that the exhaust ports & bolt holes etc are the same for a 352 and a 390, so headers and manifolds are interchangable. You are also correct to be suspicious of using pickup headers and manifolds on your galaxie - the passenger side pickup manifold does not fit, it turns downwards and hits the frame at the upper control arm, I believe.
To get those manifolds off I suggest you get out your penetrating fluid and twice a day for a couple of days go around and spray the bolts and give them a few taps to set up a vibration to help the fluid creep in the threads. Then with a screwdriver and hammer bend back the (3?) tabs on the locking washers and the bolts should come out. Mine did, anyways. I say clean em up and reuse them.
Underneath, be spraying the fasteners top and bottom where the manifolds connect to the exhaust pipe. Of course you will have the car up on a ramp or jack stands so get an extension set for your ratchet so you can reach those nuts up in there. If they are original, consider a trip to the muffler shop where they will detach the exhaust pipe for $20-40 a side depending on if you broke off a stud (which is probably why you are there). It seems possible that you could replace the gasket with the manifold still attached to the exhaust but I don't know if that would work out for you in the end.
