I can tell from the letters that Sisler knew enough to be dangerous and Clem pretty much told him to "let us do it" and we'll do it right. Some of the dialog about chrome lifters and steel cams - pretty much tells you there can be some issues with the setup if that is what Sisler believes he needs to run. Also, given all the requirements that Sisler has (which I can understand), he really needed somebody like CT to build him a complete engine, dyno test it and deliver the entire package - but that would have been some serious money (then or now!).
My guess is that Sisler never sent the heads to Clem and maybe didn't know enough to get all of it done with his own methods . . . and the engine probably never came back together (or continued to have issues). Clem was correct in not trying to just "send parts" (for the heads) - as the result is usually not good (especially with an Ardun and all the little tricks it takes to get them correctly running).
Did you ever find out where the engine actually went? Do you know the serial numbers for the heads?