Oik Pressure?

I bought a 1991 Ford 150 with 302 it has around 158,000 miles. When I bought thwey told me it had been sitting for years. Once the engine was running again I had to starting replacing gaskets. Once I thought I had them all I was driving and after about 45 mins of driving the oil pressure will down to the low mark, but it only does it at about 500-600 rpms. If I turn the idle up to about 700 rpms I see some movment but the pressure doesn't stay in the mid range It goes to the "N" on normal. I replaced the oil pump and the rear main seal this weekend. I let it sit and idle in park it idles at 900-1000 rpm no difference in oil pressure, but as soon as I put it into gear the pressure downs with rpms. I do not hear any knocking or lifters ticking. The rod bearing felt like they had little to no play at all in them. I am out of ideas the motor is strong and seems sound I hate replace a good motor. PLEASE any ideals will be great and galdly listened too.