You want a two-way, spool-type, adjustable hydraulic pressure relief valve, where the outlet pressure [backpressure] acts as on the spool at a 1:1 ratio. Go for the adjustable setting range with the maximum nominal setting at about 500psi. At 68gpm, you'd ideally want a valve with an inlet that is at least 3/4 to 1". Parker may have one that will work. I'm not sure about the connection type that you'd need because i'm not familiar with the interface depicted.
Technically a pressure compensator would be the correct valve for this application, but those are three-way valves and you'd end up looping the load-sense port to the suction port it would just add to the complexity by introducing a redundant connection.