It doesn't really take a whole lot of wear on the bearings in the engine to effect oil pressure.. Even on a fresh engine, loose bearing fit can cause low pressure at idle. Not just the main and rod bearings, but the cam bearings are probably just a bit loose causing the low pressure at idle. As long as it picks up when you touch the gas pedal, think it's probably something that you can live with. Heck, if you're eventually replacing it with a 460 think I'd just run it as you are doing.... Had a 460 in a pickup a number of years back that had the same issue on low pressure at idle, used it and abused it as a work truck/hauling truck for another 5 or 6 years and the engine held together just fine....

Anyway, one Blue Oval guy to another, sure would like to see some pics of the car! One of my favorite years and body designs!