I was always told to match the gasket to the intake not the head, your stock heads should have the outer two bolt holes but it will seal up ok

Most all intakes are blocked here anyway. Having the gasket block it off also will keep this area of the blocked ie;solid part of intake from corroding.

I have never seen a intake for a BBC that wasn't blocked off in the back... If you know of one... can you show me it? Or tell what they used them on. That would be a new one on me. I always drill and tap back there for water pressure sender, and have done both sides back there for steam pocket bleed offs.

My buddy just went through this where the salt had actually seaped through the gasket and started corroding the intake. When he put it back together his mechanic suggested they use the gasket that blocked it off and also epoxy painted the intake to provide further protection...

I wasn't gonna go into that detail but since you asked.
Edit in: I see you mod your intakes for this too.

There's some intakes with threaded in ports just over these cyl head rear coolant ports so you can run water lines back to the t-stat housing. Some come with bosses you must drill and tap to do this. Some don't have the option at all.

Here's one intake with the drilled/tapped bosses (from the manufacturer) for rear water outlet.



And another:

I prefer the ultra seal's from Mr Gasket...

Anybody use these gaskets from Flat Out gaskets yet ?

It's about time someone makes modern type gaskets for the SBC and BBC !

Full PDF catalog for all their stuff: https://www.flatoutgroup.com/docs/20...alog-Cover.pdf

The high rise alum w/ brass inserts used for the MAG motors has the drilled & tapped ports in back. Got one sitting on the shelf.