I don't know what you'd shim... other than the PTO pressure regulating valve. Even that will not gain you anything other than a day's work you don't want.
Basically... what you normally do is tee into the DP line or the pump manifold for that line and test with the DP in high and low, the PTO engaged and disengaged and the FWD engaged and disengaged. That will usually tell you where the leak is. If not, isolate the PTO from the front lines and test that separate... then add the DP to the mix... If that doesn't find it for certain, remove the DP and isolate the FWD clutch with the PTO.
I suspect if the first guy tested it and said the PTO needs seals... it needs seals.
Also... when you get it all back together, make sure you retest the pressure because I've found a couple of them now that don't and probably never did make the stated pressures in the manual of 160-180 psi in that circuit. Also beware that a PTO clutch reseal REQUIRES a rear split to get at the PTO. Tractors without a load monitor could have the PTO removed out the top... but 7710's generally all had load monitors.