True, many websters have been built and run. The guy mentioned has just finished another build log which I found quite infuriating - he didn't seem to listen to advice, even from people with vast experience of this, then he'd stumble across a solution in a later post and claim he'd solved the problem himself, whereas if he'd listened, he wouldn't have got into the predicament in the first place - anyway, rant over!
Out of interest though, how much clearance does it allow for an aluminium piston in cast iron cylinder? It must be right or they wouldn't run properly. When I tried that in my flame gulper it locked solid after a couple of mins - that is why you need rings with that method and presumably a hefty clearance built in. With iron and iron the expansion rate is the same, so you can go with a minimal clerance - that will also reduce friction compared to something with rings - more useful in hit & miss type applications where you want it to coast freely, shouldn't be a problem with yours.
Nick