Get rid of the 3-blade. It will slow you down unless it is designed to match the engine/airframe combination.
A standard 2-blade 12x6P or 11x8P will work well on that engine. You want to get the rpm up a bit - allow the engien to do its job.
Look at what is available from Graupner, APC
http://www.hobby-lobby.com/11x8_prop_left_382_prd1.htm
APC makes a 13x6 pusher that will work well on the ST75. You can trim the diameter down to about 12" to get the rpm and speed up just a touch.
Zinger makes the widest range of pusher props out there...
http://www.zingerpropeller.com/Pusher.htm
You may want to try their 12x6 pusher prop on the ST75