How much exhaust timing does it have? I'd start there first and get it to at least 180 with shims. Once there, you might find that the compression is also low enough. It seems like a 6x6.5 would be pretty close to ideal, but you could take a MAS 7x6 and chop it to 6 inches to see what happens. Ideally, Eliminator would have a selection in this size range. You might find that the engine will tolerate a lighter and thinner CF prop and make it possible to turn a 6x7?
The litmus test is making the power without the out of control heat build up....it can be done.
This is why I feel you can ultimately make more power with higher oil content. I have used 20-25% straight castor in the higher revving engines with good results in hot weather, or mixed with Klotz. I haven't heard of any optimum ratio of castor to synthetic....the engines land cool and the planes are a goopy mess.