I have the WM 60 Mustang with the OS 91 FX and was trying to figure which prop would be best, What RPM is ideal for this engine? If someone could tell me what works for them I would apreaciate it.

For high RPM sport flying and general warbird style flying the 13x6 APC will work great. (But they look a bit small on a warbird IMO.)For slower flying with less RPM you could go with about any 14 sized prop. I have used 14x6 , 14x7 and 14x4-W. For 3/D go with the 15x6 APC. I've flown my Spitfire with this prop and had great slow speed torque handling but lost some RPMs.
I'm getting my VectorFlight Edge-540 setup with the 91FX as I write this. I was running a Saito-100 with the 15x6 APC and it was plenty for general sport flying but I think the 91 will give me alot more speed and vertical performance in an 8.LB plane.
Besides it's not really a 3/D style plane.

I use a 15x8 Menz on my 91FX mounted in a 8 lb Decathlon.
I get 88-8900 rpm peak and the engine seems to like that rpm.
APC props will run a little more rpm than a Menz.
If I wanted to go a little faster I would try a 14x10 APC.
Good Luck
John

Thanks to all for thier help I put a APC 14-6 on tonight and flew the maden flight on the Mustang I tached it at over 11000 rpm so I will probally think about getting a 14-8. I didnt want to try to push it to hard on the first flight but I could tell it has excellent power, Thanks again.

APC makes a 14x7 that I think you'll really like on that Stang.