Here's what you need:
- APC 7.2x8.6 racing props (model# LP07286) for best speed, OR
APC 7.4x8.0 Q40 racing props (model# LP07480C) for a more solid take-off and still good speed.
- 0%-nitro FAI fuel (or headshims to run 5%)
- McCoy MC-9 or Hanger9 #9 Ducted-Fan Glow Plugs (buy several)
These will give you a good starting point for your GRRT. Mine loves them. Turns the 7.2x8.6 over 19k on the ground and really unloads in the air (guesstimate ~22k).
This engine is not designed for nitro, and basically hates it (blew expensive glow-plugs every flight on my buddies fairly new GRRT on Wildcat Jet-A (5% nitro 23% oil)). You can add head-shims to lower the compression ratio and allow it to run some nitro... but even then I wouldn't recommend more than 5% without a lot of experimentation.
One thing about this engine... it is sensitive to over-propping and won't come "up on the pipe" if you have too big of a prop (or have it running too rich). You can really tell when this engine "comes up on the pipe"... as the RPM's will jump from ~14k to ~19k with basically nothing in between. Kinda difficult to tune it for bigger props by moving the pipe out, there is no silicon-coupler as in most tuned-pipe installs.
When ready to fly it, you will want to richen the high-end till it just barely comes up on the pipe when on the ground. If you don't, it will lean out when it unloads in the air and your first flight will be powered for only the first 100' or so. If you richen it too much and it doesn't come up on the pipe at all, the 7.2x8.6 will not be enough thrust for take-off. Don't ask me how I know these two things.
I hope this proves useful to you!