I know you are fond of T-Bird II's, but the route is there so just deal with it. I wouldn't refin the bass or do any radical moves to make the bridge pickup go away. Just put a chrome pickup in there (mine , Lull's, or whatever) and either ignore it, or, you just may find that the bridge pickup is useful! LOL I would have NOTHING but a IV, played 'em for 30 years, just my preference.... Just remember that filled routes NEVER totally disappear unless you put a 'veneer' over it so it will ALWAYS be there. I'd keep it a IV with the PROPER hardware and pickup installed and just don't use the bridge pickup.
If a IV and the bridge pickup is that much of a turnoff, dump the thing to someone who likes the double pickup model (for more $$$) and buy a genuine bonifide II. My .02.