Hi Tstar, welcome to the forum!
The first solution that comes to mind is to replace the relief valve with the one in the old pump.
If the tractor is from 1974 it is most likely a 3000, 3600 was released in 1975. Anyhow, if it is a 3600 with cab, the relief pressure should be 850 psi. If it is a 3600 without cab, or a 3000, the relief pressure should be 650 psi.
There is a way to adjust the relief pressure by fiddling with shims in the relief valve. That is an ordeal, since the valve is inside the reservoir and you have to drain the oil each time you adjust the shims, put the cover back, fill with oil, test the pressure and (probably) do the thing again until you are satisfied. Besides, I do not know if it is possible to lower the settings that much on your pump with just the shims.
If the old relief valve can not be used, I would put an adjustable inline relief valve with a pressure gauge on the pressure line. Cheaper than buying another pump, and you will have full control.