Well, if you can find the piston info on the build info, that would be imporant to complete the picture of what you have. The reason I question that it is a 'stock' 340 bottom end is that the stock 340 pistons stick up about .018" above the deck. The Speedmaster heads with 64 cc chambers (assuming that is what they are) are closed chamber heads. You would need a thick head gasket to keep the heads' closed chamber area far enough from the pistons to keep them from hitting each other; a standard Felpro head gasket would juuuust be barely enough and we don't know that it was used or something different or thicker....or thinner, in which case you may be skirting disaster.
I don't want you to rebuild your bottom end, but we really SHOULD get a handle on what you have for compression ratio and that is all in the pistons and head gaskets and head volume. At least I think we know the head you have at this point.
One of your posts seems to indicate that this engine has to rev out before it starts making good HP; one cause of this is a combination of too much cam with too little compression ratio. So that is why the bottom end details are needed to make a complete picture of what you have.
I know you would like a list of things to just change but no one here is going to be able to help you do anything for certain, but just take random shots at this or that change without complete info. We are getting some info but not all so the help is going to be limited as to how accurate it will be.
Got any pix of inside the valve cover? Or under the intake manifold?