Looking for any recommendations on exhaust gaskets for 400 HO with long branch. The ones in prior pretty much disintegrated.

The stock steel ones have always worked fine for me. Just make sure to wire brush both surfaces and install correctly. You put the two end bolts in, then drop the slotted ends of the gaskets down onto the bolts between the manifold and head. Slowly insert the remaining bolts and once they are all in place correctly, tighten to specs. Rock Auto has them all, both Ram Air and regular. Pontiac exhaust gaskets I take it a step further and use stainless steel bolts with a dab of anti-seize. You may need them to come off some day.

Those steel shim gaskets work ok if both mating surfaces are flat and smooth. Problem over the years is the exhaust manifolds warp and rust and get pitted.

I have been fortunate that my manifolds and heads are pretty flat and smooth. You can dress them with a sanding disc. Or use a schmear of high temp silicone to help seal. Even the 1971-72 manifolds/heads that are missing the end bolt holes can be used if you bevel the surface on an angle to press tightly against each other. Worst case you can use soft header gaskets. Normally the originals are just rusted away as the OP mentioned. So far the steel ones have worked well for me.

We just happen to be doing the stock manifolds on a 1978 Trans Am. We yanked the idiotic headers off, tossed the entire stupid loud system, and are restoring the factory parts. I cleaned and painted the manifolds with Eastwood Stainless Steel paint. Nothing lasts but this lasts longest. The flange surfaces were in very good condition. It took only a bit of wire brushing to get them ready. See the bolts on the ends. Stainless steel. The steel gasket rests on them so leave the manifold loose enough to slip them in. Then insert the rest of the bolts being careful not to hurt the gasket. If they won't go, adjust the manifold/gasket gently until they do. Once all the bolts are in, tighten to specs. I had to bend the dipstick tube back where it belonged. The header gaskets had caused the tube to be bent out of the way. Another plus to factory gaskets.

All buttoned up and not a single leak. The factory gaskets work great as long as you make sure the surfaces are flat. I forget what size bolts went with the long branch manifolds. These stock manifolds took 3/8-16 stainless bolts that were 1 and 3/4 inches long and 3/8" flat stainless washers. Grainger had them at a good price.