It has been a very long time since I farted about with flatheads, but I do remember they started out with iron pistons and 4 rings per. Last one I messed with was a 38 so it had all the old school stuff.

One thing I do remember is that you should replace the water distribution tube in the block. They tend to rot out and cause funny cooling issues.

As for eth side seals, any chance of just using RTV? Not a real fan of that stuff generally but sometimes it does the trick.