On my old Six's, I would fix the leaky broken Exhaust manifolds, and then (sometimes) I'd wind up with the terrible Putt's. I lived in Alaska at the time and had three of these trucks, over a 12 year period.
It's two things:

One, The lousy exhaust manifold bolt repairs we are sorta forced into doing.
If you break a bolt, or someone before you has broken a bolt, you should really pull the Head and do a good fix.
IF you dont, that bolt will never hold well enough to stop the Putt's when you are under power.

Two: A combination of warped manifolds and the head surface being un-even at the Junction. Sometimes it's the Header pipe leaking at the manifold/pipe junction.

The FIX:
Bite the bullet.....get the pain overwith.
Take the Head to an Auto machinest shop. He'll square it up.
The Manifold is more difficult, but the same guy fixes it.
Put the Manifolds together on a good head. No Gaskets.
Now...Make sure the two manifolds are solidly bolted together with good thread holds, and the Gasket between the two is present.
Take both parts to the Machine shop and he'll surface the manifold for you.

From experience... this works! And it was less than $100. back in the 80's.
Boy was it nice to hear nothing but 'white noise' coming from under the hood.

My Suggestion on preventing those cracking manifolds:
Maybe we could make them last longer if we had a 'chimney' around the very hot part, below the carb.
That would put the COLD/HOT junction down below the manifold. Somewhere in the Exhaust Header.

I think the cracks appear because that part of the manifold gets so much hotter than the rest of it, and then cools very quickly when the engine is shut off.
Guys that live in the northern climes know about this litttle atrocity because it happens to lots of other things.

So; if we could build a sheet metal covering around that part of the manifold. Nearly air tight, that would extend down the Header pipe, at least six inches.
Now the part of the manifold that cracks, would cool down much slower.
This should move that COLD/HOT junction down onto the header pipe which can stand that kind of stress.

How about a manifold made out of Boiler Plate? Would that be better than the Cast of the stock manifold?
So what do you guys think? Does anybody else have an opinion on this?

 

I slowed down the loosening of the front of my manifold by using the longest grade 8 bolt and some loctite etc. The stud that was in there was in by just a few threads, so no wonder it didn't hold.

I am thinking of installing EFI manifolds. The 3 into 1 design would seem to be less prone to the twist and shout action of the longfellow.

I understand some tweaking is required, and the fabrication of hot air and thermactor stoves.

Best part: My smog guy told me if he sees "Ford" on the part, it's good enough for him on the visual.

 

I theorize there was a reason that Ford originally specified that the exhaust manifold was to have no gasket at all. The gasket that folks put in there reduces the heat transfer from the hot exhaust manifold back to the *realtively* cool heads (which have water in them of course). That causes excessive heat buildup in the exhaust manifold and ultimately leads to warp and cracks.

I think one should try to get a good seal with no gasket at all on the exhaust side, just a thin layer of the hightemp copper stuff. The original shop manual very carefully describes using a gasket between the intake and exhuast (at heat riser), as well as for the intake to head surface. But it mentions only "light coat of silicon grease" for the remaining surfaces.

The aforementioned proceedure of bolting the intake and exhaust together on a known good head (as if I had one laying around spare!) and then having a machine shop true the surface, will not work using the factory prescribed install proceedure, as the intake and exhaust surfaces are slighly out of plane, due to the fact that the intake has a gasket and the exhaust does not. Follow that??

I do think that for an exhaust that cannot be made to be tight without a gasket, THEN the idea of taking the bolted together exhaust and intake to a machine shop for trueing up is a good one. BUT, afterwards remember to use only the single gasket that goes under BOTH manifolds. This gasket is not original, but most gasket sets for 240/300 have it, precisely because of all the leaks that folks have with 300 engines.

 

That was a good reply CDHerman.
I did omit the fact that I used an aftermarket gasket that served both intake and exhaust.
I had the manifold set trued by the machinest and the end result was absolutly fantastic.
The important part is to loosen the four bolts holding the two manifolds together at the heat riser, then tighten the manifold to head junction in sequence so that the two parts are aligned.
Then tighten the four bolts at the heatriser junction.

Do this on the trued head that you just retrieved from the machinest.
Or take the manifold to the machine shop and do it there, but you get the idea. True it all and it will work quietly.

I always suspected the flow is restricted too much for the kind of seals the ford 300 has at the manifolds.