RE: O.S. 1.60FX Pusher
The stock muffler will give you plenty of pressure for the fuel tank. As far as a 3-blade prop goes, just take the 2-blade spec and either drop down one inch in diameter or one inch in pitch.
Now, the problem won't be to find a 3-blade prop, but a 3-blade PUSHER prop! Usually, a 3-blade prop won't work any better than a 2-blade prop, but it can help a bit with ground clearance.