Honestly,
These Bluesbirds would have been more appealing to me in the first place if Guild had used HB-1s in them... I don't see any reason why they wouldn't sound great, either!
I
am curious about the swapability, though. I know that Guild HB-1s are wider in the dimension that's parallel to the strings' path than Gibson humbuckers. That having been said, I can also imagine that Guild factory would have had templates on hand already for routing holes for HB-1s and may not have seen a need to make new, smaller ones for the BBs and S-100 reissues that had Duncans instead. If I remember correctly, the rings for the BBs had a two-screw adjust on the bass side and the Seymour Duncan pickups had an adapter plate on the height-adjust foot.
You might PM Murkat, who had a
thread about one in the FS section a while back. To summarize, he found the adjust feet to be too large on the HB-1s to drop in, but later ended up installing them in the BB. Thread did not mention how he got around the issue, but I'd guess by filing down the feet of the HB-1s...
If there is a conflict w/ the route, he should be a good source of advice, since he clearly solved it. I suppose he could also advise you on how it sounded, but I'm not sure there should be much doubt in that department...