I'd swap a HV pump on there and see what happens. My guess is that you have an internal leak so the standard pump just isn't building a lot of pressure. Could be the EDM lifters or it could be something else. This is a good reason to do the prime and oil pressure test before you put the intake manifold on. You could also put a high pressure spring in your standard pump. That may or may not work but it is cheap and easy. If you need more volume then the heavier spring won't fix it but it will fix the problem if your existing spring is softer than normal. For a really cheap and easy solution just slide a couple of washers behind the spring that you have right now and see what happens. A lot of times I'll use a standard spring with a shim or two rather than step up to the heavy spring. I keep a selection of springs and pumps on hand so I can tune the oil pressure on each engine I build. Sometimes I use a standard pump with a heavy spring, sometimes a HV pump with a soft spring. Sometimes some other combination.