adding a second pickup to Gibson Thunderbird II

I just bought a second Gibson Thunderbird II (60's non-reverse) that's had its finish stripped, so I'm not worried about keeping it stock (my other T-Bird II is mint - I'm not touching it). I'm interested in adding a second pickup to it for a bit more versatility, but haven't had the opportunity to try really testdrive any 2 pickup T-Birds (other than for 5 minutes at GC at low volume). Does anyone have advice on what a good second pickup would be, or should I just get another T-Bird pickup? My inclination is to just add a second T-bird pickup, but if there's another pickup that might work, I'd like input. I LOVE the sound of my T-Bird - I don't want to completely change the sound or anything, just a bit more variation.