I have converted and flown that engine on two planes and I like the 18x6 Master Airscrew Classic and the 18x6W APC on it. It will swing a 18x8 Zinger Pro but it loads it down a bit. If this is a swamp buggy go with the 18x6 and you will be very happy. You want to move a lot of air and the 18" prop will do just that. A 16" prop is too small for that engine IMO.