When you say the 40HB is the only engine you have ,tells me your not wanting to buy a jet or turbo fan setup because of expense. Most guys don't read between the lines and money is no object. Provided you have a long enough prop shaft you can put 2 of these togeather with 2 small alignment pins drilled in the hub half way between shaft hole and where blade starts for the best strengh. Don't glue (waste) the prop back hub and washer or nut will keep pins in place. This set up should yeild a 12x4 pusher prop load perfect for a 40 HB.
http://www.masterairscrew.com/index....WPROD&ProdID=3
That prop inefficentcy at the RPM you will turn is HOGWASH. You'll now have a 6 blade pusher .