So after I logged off, having just replied to Waz... I went and took another look at the bass. The "rosewood" fingerboard is 90%+ of what overhangs the neck pickup rout... So I actually could have a humbucker in there if I sliced off the overhang...and the scale would still put the mid-pup in the p-bass location.

So I did another search, and found another one of these EB50 pickups had just been listed on eBay, from the same seller, in fact. So...I bought that one too. Hope it sounds good... Will probably have to trim some pup rings for the neck position, but it will definitely fit.