I'd strongly suggest that you take it to the best tech in town and let them advise you. From what you say it seems that someone has put a (P90?) single coil at the neck and a replacement humbucker at the brige - especially if you cant get it low enough.
What i would do is to check the serial number, get it valued
and then have it changed back to the original (type of)pickups and either get it seriously insured or sell it ;)
I'm suggesting this mainly because firstly old Gibsons have a great desirabilty (probably because the necks break easilly),secondly they usually have fantastic tone because of the (higher?) quality woods used (in combination with their electronics) and thirdly because i'm jealous :D.
Tony Iommi, I beleive, used a standard SG in Black Sabbath so i guess you've got a good metal tone with standard pickups. Also a certain Mr Young from a small Australian popular beat combo??
I'm guessing that their guitars weren't heavilly modified when they started with them.
I'm guessing the stock PU equivalents will be Gibson 496/490/498 "Paf" humbuckers.
Sounds a nice piece.
Good Luck.
Accuracy,you say? hmm interesting concept..