You should have a relief valve in the valve body. If there isn't one, add a separate one at the pump outlet ond dump its return to the pump inlet.
What the relief pressure should be is an open question, enough to do your work and not so much that you risk breaking the tractor. For GT FEL's, anything over 1000 psi puts a high risk to the tractor. Sometimes you get stuck and have one with a 1500 psi minimum and the rear tires become airborn when you try for too much payload. It's an adventure trying to figure out what the tractor's limits are before parts break. I broke both spindles off the front axle, the left one twice, finding the limits. After rewelding the spindles better on the 1655, I haven't broken anything that wasn't the direct result of my laziness. I didn't bother to plug and weld the hole through the frame on the left side when I notched it for the starter of the replacement engine. The frame broke from the notch and through that hole.
Having said all that, plug the hole in the left frame member of your 1855 and improve the weld for the spindles and 1500 psi is surviveable. I've had the rear wheels in the air on many occassions with 650 lb of ballast, and have lifted and transported, on level asphalt, a 1250 lb payload. The front tires were hugely overloaded and wouldn't climb the 1.5" from the asphalt to the garage floor. The 1655/1855 tractors are very tough machines.
That pump is an ideal size when run off the 2000 rpm PTO. I'm not fussy about runing 5.5 hp with a 1/2" shaft, though. I've twisted off a 7/16" pump shaft, but it did last several hundred hours of my mistreatment.