Thanks RIB-Teccie, this is great information.
I'd not heard of the 60d, must be very rare. I know the 70CES is very sought after, although I think it's a bit heavy for a thundercat.
It seems to me I want the 60 gearbox? which will give higher top speed but less acceleration.
Think the hard part is finding a short shaft one with the 1.71 (14:24) ratio gearbox.
Not much point changing to a 1.84 gearbox, the one on my tohatsu 50 is 1.85 and I'll be able to spin it to over 7000rpm.
Think the 55's use the same props as well, 13 spline.
I don't think you can have too much power on a thundercat, as long as the engine is light enough.
Is yours short shaft?