I think that once you have fitted a Ford or Mazda box used to work in classics and kit cars, the options on ratio's are almost limitless due to the amount of people using and racing with them. There are lot's of specialists around like this one
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I'm sure that In Racing can supply a Bristol bell housing to start with, or I may have one if needed ( shame to break up a good box ) I don't think you would get as many ratio options with the overdrive route.
In Racing make new Bristol engines and support a lot of race cars so probably best for advise due to the knowledge of the engine characteristics. Although if it's down to budget and race ware maintenance, I think the Ford / Mazda box route will win.
Good luck