I'm in awe Spud. What you have done with the OHCP is remarkable.
I've only done some 15 passes with my digger (half those were 1/2 track) and the engine has been rebuilt 4 times! Maybe I'm doing this all wrong lol.
Currently running a 468 donovan, 110 pump, your SM5 (8 amp), and 3.7 gears. 8000 RPM
I've been thinking about your success........maybe a 2.90 gear to get the revs down further, and a bigger pump to make up for the lower pump speed (my 110 enderlie now has a blank main and 180 psi), and install a Supermag +.
I'm carrying an extra 200 lbs weight over your digger, but that shouldnt hurt too much.
Cheers mate
Graham