The size of the
oil pan should have no effect on the
oil level required to ensure the
oil pickup is covered with oil. The mark on the dipstick is calibrated to place the top of the oil level in the pan at an appropriate height to ensure the oil pickup is covered under all driving conditions. adding more oil
underneath the mark has no effect on this. If you have any questions, call Bill or Lewis at Southern Automotive and talk to them about it.
BTW, you may wish to ask them about the possibility of driving the car with a dummy oil dipstick in the engine and carry an actual one for measuring the oil level. I understand that under extreme rpm operation, it is possible to pull the stock dipstick into contact with the rotating components in the bottom of the engine. Many of the FE folks take a stock dipstick and cut it off so it doesn't extend very far into the tube, and that seals the tube for operation. Then carry an extra dipstick in the trunk for actually measuring the oil level. Again, ask Bill or Lewis.
As to whether or not oil pressure is an indicator as to whether or not you should add oil, I'd suggest that it is not, until you notice that the oil pressure is holding below 25 psi under normal cruise. At that point, you're about four quarts low and you're getting intermittent oil starvation to the pickup, and hence, the bearings. Hopefully, you'll not let it get to this point.
For reference, my Southern Automotive 484 stroker starts with about 62 psi, and I see this decrease to about 53-56 after the engine has warmed up to operating temperature and I'm running above 1500 rpm. When warmed up, at idle (900 rpm) the oil pressure sits about 32 psi.