Something to think about: a rebuilt gearbox from Canley Classics for a GT6 is currently around $400 (on today's exchange rate). Granted, shipping won't be cheap, but if my experience with a Spitfire gearbox about 10 years ago is any indication, the quality is as good as it gets. By comparison, Spitbits wants $775 for a rebuilt with exchange of your old core (or an extra $150 if you don't have one). As odd as it seems, you might find that even with shipping from the UK, you'll come out ahead with a Canley rebuilt.
Meanwhile, I'm sure lots of folks will rave about a five-speed conversion, and they might be right about that. I don't know. I do know that years ago, I put many thousands of miles on first a GT6+ and then a GT6 Mk3. I never had a problem with either box (both cars were used -- the Mk3 very used -- when I got them), and I never mourned the lack of OD or a fifth gear. With 20 mph per 1000 rpm in top gear, that torquey and very flexible six isn't exactly screaming even @ 70 mph. (Full disclosure: I don't even mind my Herald's 4000+ rpm @ 60-65 mph, since it seems happy to do it all day, every day if I want!)
I don't think value would be affected one way or the other by stock or five-speed conversion gearbox.