There is plenty of prop clearance for an 18" prop on the Phoenix. The problem is gear length. You will need to make longer gear. I am running 12-6s on the OSs and in a cross wind with a wing low landing, I scraped the tip of the upwind prop. You'll just need to bend some 3/16 wire.

I had planned on using Saito .91s in mine, but my flying buddy, Flaps Laffert, has them in a not yet flown R4Q (Marine C-119) model of a plane that he flew in the Marine Reserves in the late 1940s & 1950s.

A point of interest:

Bill Robison and I are in the same club. I pass his place on the way to the field. We rarely see each other or fly together since I wake up at 5AM (sometimes I sleep in till 6), pick Flaps up (he's 84) and head to the field, while Bill flies late in the afternoon. Bill is one of those late night people and I crash at 9PM. We do talk on the phone.