[python] Request for advice.

Siwmae Pythonauts!
I need to get some opinions about a construction decision that I have to make now.
I'm using a BMX frame to donate parts for my python, and I'm wondering about using a pair of seat-stays, intact, with V-tech brakes already in place, so that I wouldn't need to fit new lugs, dropouts, etc. (For anyone unfamiliar with GB terminology, seat-stays are the pair of frame tubes that go down from the seat tube, just under the saddle, to the back axle dropouts on a standard bike)
My question concerns the placing of the articulation pivot (steering BB). I'll be using a standard bottom-bracket shell, with sealed-cartridge insert.
Leaving my donor seat-stays just as they are now, without shortening them at all, I can use a short length of the seat tube left as originally welded to the stays, and slit open slightly, to act as a cradle to locate this BB assembly at the right vertical angle (about 67 degrees), but it then places the articulation pivot quite far back from the edge of the front tyre.
I reckon that the axis of the hinge would be about 12.5 to 13 cms back from the outermost surface of the front tyre, measuring horizontally forward from the top of the BB spindle. 

I'm using 20 inch/51 cms wheels.
You can get some idea of how I intend the frame shape to be by going to Dirk's site, opening the 'python for marc' page, and looking at this slide in the gallery: 

http://dirkb.dnsalias.net/image/pythonmarc/20050908.07.hires.jpg
where Marc is trying out sitting on the bike for the first time (with a big poster of a very fine looking blonde girl on the wall of the workshop just behind him).
In that picture, the line through Marc's hip joints seems to go dead across the axis of articulation of the bike, which I believe is good for avoiding PSI.
In the picture, the top of the steering pivot (actually a head tube and a front-fork tube, in Marc's case) seems to be 8 to 10 cms behind the outer surface of the tyre, possibly less.
So can I get away with stretching that measurement to up to 13 cm. That's the question. It will save me some tricky cut-and-weld work if I can. I was still intending to arrange the seat -- a copy of Jurgen's originals, with safety-belt webbing -- to place my hip axis dead across the articulation axis, as with Marc's bike.
Any opinions please? Am I going to make a difficult-to-steer frame geometry if I do this? 

Diolch yn fawr (Great thanks),      Rhisiart.
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