RE: prop size
It's hard to find a lot of info on the Laker, it's an old Laddie Mikulasko design from RCM Magazine. The listing on RCM plans calls for a 40-46, and I'd bet that Laddie put that engine pod down low so that an 11" prop is probably as big as you can go.
You might want to think about a 46AX, or even electric power. I used a Turnigy G46-670 turning an MAS 11x7 3 blade on five cells in a Mikulasko "Arrow" flying boat, and it wails. The nice thing about electrics in "pod engine" flying boats is that the battery pack makes it a lot easier to balance the dang things. They really want to come out tail heavy.