Couple of questions. First how large is the oil tank your using? Second, how long do you intend to leave the system in operation after you start the engine? Reasons I ask are this. Dumping oil dumping off through an orfice, relief valve, etc means that the oil isn't doing any work. That being the case all the restriction does is create heat in the system. It doesn't take long dumping even a gallon a minute off to create ALOT of heat in a hydraulic system. If the tank doesn't hold enough volume for the oil to hang around and cool off before being circulated across the restriction over, and over, and over again within a short period of time, the heat will accumulate even faster. Ultimately you'll overheat the system to the degree that it will break down the additives in the oil, and cause premature pump failure, seal failure in the cylinder, etc. Typically the rule is that you want a tank with a capacity at least twice the volume being pumped. In your case, with such a small pump/system to start with, it would really need to be a bit larger to prevent the chance of overheating if you plan to leave the engine running for any length of time.
That said the right way to do what your wanting is to install a simple flow control valve in the system. Doing this will make the full flow from the pump usable, but also allow you to properly meter the needed flow and allow the excess flow to return to the tank. This will allow you to run the engine at a higher speed, which will give it the necessary HP, and some cushion in that department (ie-allow the governor to work properly), to handle the heaviest loads (ie-the highest required system pressures), and not lug, or stall the engine, or slow down the flow as a result of said lugging or stalling. In other words the valve will allow the same GPM across it at 1500 psi as it will at 3000 psi while the flow across a true, fixed orifice size will usually vary with the pressure. Also the valve in the link below has a relief valve integrated into it so even if it were to get shut completely the pump wouldn't dead head and be damaged as a result. With a small, fixed orifice, the chance of dead heading the pump due to a piece of trash in the system could create a potentially hazardous situation given that piston pumps are considered to be positive diaplacement, and many are capabile of pressures into the 6000 psi range. If you have a relief already in the system that wouldn't be an issue, if not it's something to think about.
Good luck and post some pics so we can see the setup when your done with it.