I put on Macs bolt on muffler on my ST 75.
http://www3.towerhobbies.com/cgi-bin...?&I=LXZ657&P=7
Currently in stock.
Which I side mounted on a Great Plane Big Stik 60 built from a kit.
Still need to cover the wing and install the radio.
A 13-5 is a great prop for the ST75. It should turn about 12,000 with that prop and give unlimited vertical