If you could split the log with a single wedge & a sledge, then 1200 psi would be adequate with a 4" bore cylinder. I have the relief valve on my 4" bore log splitter set to 1250, and the only logs that stop it at this pressure are the ones with big knots. For these logs, I have a pressure intensifier on the splitter that will send about 5,000 psi to the cylinder at 1250 pump pressure. I rarely see over 2500 psi on the largest knots; but at only 1" thick, the splitting wedge acts more like a knife and actually cuts the knots in half.

Your biggest problem using a steering pump will not be the pressure, but the volume. My splitter uses an 11 Gallon Per Minute pump and strokes the 4" bore cylinder 24 inches in about 7 seconds. I doubt that steering pumps for these trucks flow more than 3 GPM max; probably more like 1 GPM at an idle. That is over a minute per split. Add at least another 30 seconds to retract the cylinder and a few seconds to position the log, and you are looking at about 5 minutes total just to quarter a round. It would take about 100 10" diameter logs 24" long to make a chord. We are talking about 8 hours just to split a chord of wood. Maybe someone here will be able to tell you how long a FE will idle on a gallon of gas, but something tells me that you will burn up about as much energy in gas as the energy you will get out of the wood.

If you have the NP435 tranny, a better alternative would be a PTO pump of addequate size and about a 5 gallon oil reservoir, asumeing you really want to power the splitter with the truck's engine.