Classic/Antique Car Repair: 1948 buick 3 part exhaust manifold, gasket material, expansion and contraction

Question
the roadmaster manifold is in three parts. a center section and two "horns". the two horns slip fit into the center but require a sealer or you get blow-by. i have a service manual but this is not covered. what was used originally? what can be used now? its got to be more high temp than rtv silicone. parts houses so far have no advice. wood stove gasket cement, good to 2000 degrees, was a suggestion. can you help?

Answer
When these units were made and used in the day the three units were made for a tight slip fit. There was no gasket material or sealer used. The idea was to allow for expansion and contraction as the manifold was so long, that allowing it to slide in and out as it heated and cooled would prevent breakage and cracking. We used to use valve grinding compound to lap the fit of the manifolds together to achieve the fit that we wanted. If you have blow by then the manifolds are worn or someone along the way took a file to them to make them slip together. Using a rigid sealer will crack out due to the sliding action. If you really need a sealer then one of the stove/furnace compounds would be the choice. Ya, I was there during the day that we were dealing with these cars in the shop on a daily basis.
Brad