On budget build small blocks I used to use a holley jet. I turned the threads down just a bit and it would fit inside the spring. 70 lbs at idle usually. Don't laugh too hard guy's, it worked lol.

 

IMO- I have always had erratic pressure and other problems due to SBC oil pumps and deep pans in early Novas. Throw that SBC oil pump away and get a stock BBC oil pump. Works 10 times better. I stop using SBC pumps many years ago. If you want to mess with the pressure them get two buckets, a drill motor, an old rear main cap, some oil and oil pressure gauge and have at it. Expect to make a small mess. Too much pressure is almost as bad as not enough. You can run your pan dry and it's hard on the bearings. Also your pin can get loose from pulling in and out trying different shims and fall out in your pan and loose all pressure.