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What do you mean by particular conceit on the fit geometry?
You're choosing to place your hands, feet and arse in a particular arrangement for style reasons (or, if we're being generous, for a particular combination of ride and handling). If you don't change that, then there's very little difference that the practical differences between frames the right size will make, whether it's ±1" on the front and rear centre distances or ±1lb on the weight. That's not a particular point about this project, it applies just as much to any other kind of bike until you get into high performance riding.
suspect I'd need a longer TT
Not so much. Raising your torso angle cuts a lot out of your reach, making a short stem and swept back bars come to your hands with much the same squarish geometry as works for normally proportioned people on a road bike.
gbj_tester-
for style reasons
I am not cool enough to consider such things. I just want a bike I can ride while looking straight ahead, sat upright. All dignified, like.
±1"
There's probably 3" (7.5cm for the rest of us) difference in top tube length between a large MTB frame and a large road frame, placing the swept back bars either 7.5 cm further/closer depending on that choice. Surely one choice must be preferable to the other. I'm just not sure how to work that out. I guess I might just have to go try some Raleigh Sports type bikes and make a load of measurements.
Ndeipi
Ok I take your point about tyres. I'll choose some zippy ones. But I think that's maybe 50% of the zip. Low rotational mass (nice rims) and effective brakes are gonna add significant zip.
What do you mean by particular conceit on the fit geometry?
Edit: Did you mean concept?
I suspect I'd need a longer TT than on my road bike. Should I maybe look at MTB frames?