Scared Silly
Contributor
Unless you can find a used manifold, you are better off making these 3500psi tanks singles or selling them and getting 3/4-14 valved 3442psi steel tanks for the doubles. 7/8" manifolds (and even single valves) are stupid expensive.
Using the ebay seller I linked above one should be able to put together a 7/8" manifold for around $175-$200. I think that is dirt cheap, especially for a thermo.