RE: Which gas engine to 72" giles?
Your setup will work-extremely well.Iam flying almost identical setup --900squares - 10.2 lbs EVO 26 on their pipe (same as you describe.)
best prop - 17x8-OR 18x6 -try various blade types but prop for about 8500-9000 on the ground running a bit rich .performance wil be strong vertical instantly on take off at a fast speed. instant hover recovery -vertically and will accelerate straight up
Use 32-1 Mobil fuel
Do not peak engine on ground! once it gets up to temperature in flying -it will be correct mixture .
Iam at 4350 ft elevation (over 1000 m) so your results will be even better .
If you go to over 11 lbs (5 kilo)-performance will not be as good as noted .
Also do not use in cowl muffler -the setup will be poor in performance.