I suppose that in an ideal world the answer to John Clay's question would be to use Columbus' SPX series to get the best of the thicker gauge tubes in the SP series allied to the helicoidal reinforcement at the bracket end of the down tube, seat tube, and chainstays
The SP tubing has wall thicknesses of 0.7/1mm as opposed to the 0.6/0.9mm for the SLX. The seat and the chainstays of the SP are 1mm whereas on th SLX the seatstays are0.7mm and the chainstays are0.7/1mm.. The fork blades on the SP are1.05mm compared with 0.9 for the SLX I seem to remember from Milan Shows that many of the smaller builders in Italy such as Daccordi, Rosin,Tommasini, and Guerciottis always used the SP series for any frame larger than a 55cms C-to-C. Presumbly the same principle would apply to frames using the SLX and SPX series.
A very comprehensive Columbus catalogue from the 1985 period shows all the butts in the tubes as being progressive ie tapered rather than abrupt. The latter would probably give rise to stress raisers in the tube wall.
As for the length of the tubes for larger frames, in the UK Columbus always supplied in three different sizes depending on the size of frame to be built ie Sizez A, B, and C. John would probably need C.
In 1980 I attended a Columbus worshop practice seminar, the aim of which was to instruct builders on how to "work" and braze their sets of tubes. I remember being advised that under no circumstances should a silicon bronze rod be used, a slver solder/braze material being essential. Have you framebuilders on the List always adhered to this cardinal rule?
Norris Lockley .. Settle, UK