I have done it and the answer do they interchange is not really. There are a couple of end bolts that are half a hole out.It was no big deal I just threaded plugs in and redrilled and tapped new holes (C8AE-H heads 14 bolt). But i did it on my bench/drill press. This was over 35 years ago and I forget the details. Get some gaskets and lay them on top of each other.

The spacer does not have a butterfly in it..

Hello everyone.
I have a question I am asking on behalf of a elderly friend who owns a 68 gt390 coupe.
He has recently had the engine rebuilt and has stroked it to a 445 fe with trick flow heads and I believe blue thunder 4v aluminium intake manifold and has dressed it to look factory stock however with some upgrades like mentioned above plus aluminium water pump etc.
Now he has a pair of gt390 exhaust manifolds I know he wants to keep with exhaust manifolds and not switch to headers.
He asked me recently could he fit a pair of 428 cobra jet exhaust manifolds in place of his gt390 manifolds and if he switched what would be the hp/to over the gt390 manifolds?
I believe there have been tests comparing the 428 cobra jet manifolds vs different brand headers and just wondered if anyone on the forum here with all the extensive knowledge would know if and what the gain would be swapping  exhaust manifolds.
I believe I am correct in thinking the 428 cobra jet exhaust manifolds would fit in the tight engine bay of the 67/68 mustangs as the 428 cobra jet was released mid year and offered as a option in the 68 mustang.
Also I know there's the early 428 cobra jet exhaust manifolds that use a spacer on the passenger side manifold and the later 70 design that did away with the spacer on the passenger side. Am I right in saying he could use the 70 design cobra jet manifolds without the spacer to make things simple (as I believe he has converted his Carb to electric choke) and just then get a 70 mustang h pipe to bolt up to it and then fabricate his own exhaust system from there back out to the valance.
Many thanks for your time and any help or information  anyone can give is much appreciated.
Have a nice day.

Nice result Doug, Glad that you finaly got the engine ready to run

Ok, guess there is no easy way around this.The block needs to be decked. Now L2291A pistons sit 0.024" below deck and with -10 cc recess volume compression is 9.2.
Decking the block to 0 clearance and adding set of -5 cc pistons would bring the compression up to 10.0 - 10.2 with 0.040 gasket- perfect for this pump gas Galaxie.

Thanks for your input.

Autotec 1000778