for starters I'd go with a tee in the pressure supply line to the valve from the pump,mount the gauge to that tee..use a 5000psi gauge to give yourself some gauge headroom when reading at high pressures. Surplus Center has it all in stock for the most part,they have an awesome selection of adapters and fittings.From the looks of it this is an open center system(flow from pump thru valve,back to tank with all valves in neutral) at 2500psi or lower. relief valves are only going to actuate when they reach 'cracking pressure' or the pressure where they will open and dump back to the tank,or in this case, the valve will bypass flow back to the return line,or hose that goes back to the tank. It sounds like you have too much flow in therms of GPM, or pump flow..which is maybe why some one put a 13hp engine on it, so they can reduce flow by only running the 13hp engine at 1/2 throttle, halving the flow vs. full throttle, but still have enough power to run the pump. Try running the hoe at half throttle and see if it's more manageable to operate.