There's one of two ways I would try it:

1.) Try to drop the manifold off the head and see if you can maneuver the gasket in from the top.  Good gaskets should have two hooks on the end for the outermost bolts, which you can put back in like 2 threads once you clean everything.

2.) If there's no access that way you may have to pick up the engine to gain the needed access.  I seem to remember being told that the manifolds typically were left in the engine bay attached to the exhaust when doing an engine swap with an FE.

I've never actually done this myself, but those are the two ways I would try based on things I've heard over the years.  It doesn't look fun, and I'm sure there will be a lot of foul language involved.  Just make sure to wash your mouth out with plenty of beer. 

50vert wrote:

If the brake rebuild involves the booster and master, pull them before trying the header gaskets ....... it seems spacious without them in the way.

With the above being done try un-bolting the headers and upon re-attaching them......I like to use red RTV on the new gaskets and stick them on the exhaust ports of the engine.
Bolt the headers down fanger tight and let the RTV dry overnight.
Next day the gaskets will be in the perfect position and you won't hafta worry about them sliding and shifting while you're  fighting with installing the header boltz!
On a Mustang (SBF) loosin'ing the "Mow-der" mounts and jacking the engine up some helps when trying to start the bolts. (get them ALL started before letting the engine back down)
6s6

val fulesday wrote:

during futher inspection I found the  vac. connection at the brake booster is broken. can these on the 67 cougar be screwed out or off or are they just a pop in kind of a thing
 

 
I would think it's a plastic piece sealed with a rubber grommet that pops out.

For the header gaskets.

With the master and booster removed, you will gain some working room.
If the motor will crank, get it warm then start to loosen bolts. The top row of bolts has a tendency to freeze up after time. Generally they will round off or break attempting to loosen cold. Real GT heads are 14 bolt with no bolt holes on the bottom middle cylinders and the top outer are lower than the centers.

If the car ran with leaky headers long enough, it will make for a poor sealing surfaces.  Cause the exhaust gas has worn away the casting at the leak point.

I've had good luck with pulling one motor mount bolt, then rocking the motor up to one side. My headers (FPA's) can be wiggled out the bottom this way with the idler arm and inner tie rod disconnected from the centerlink.

I'm partial to gaskets, Felpro 1442 is a good one for the Cobra Jet 16 bolt pattern. I don't know if there is a GT 14 bolt equivalent. Lots of guys use RTV with good results.

If you buy new bolts, ARP has some with a 3/8 hex head that allow access for a ratchet wrench to the majority of them.

Good luck.