Hi Sharpnr.
I wish the 70 S II's were still available. That was the PERFECT engine for the B-25. But I know many have had success with the new FL 70 as well. I am building a new B-25 using the O.S. .81s. I haven't yet flown a B-25 with these engines. With the .70's, I was running the Master Airscrew 12 x 8 3-blade. This was the best 3-blade (of the ones I experimented with), but I think the .81 will pull this propeller even better.
Also with the .81, I will be using the exhaust pipe from the .70 Surpass because this will locate the muffler perfectly in the molded-in "divet" in the fiberglass nacelle. However, this required slight trimming of the engine mount and turning the mount to a new location.
Of course, I have also read several accounts of great performance with the Saito as well!
Here are some photos....