Hi Anthony,
Good to see the car back to it's former glory. I think you were only half way through when we saw it at the Wybunbury meet last year.
As far as the shaft goes, when I compared my 6 speed box to the 2.8 box it looked exactly as shown in the video. The only discernable difference is the extra length on the nose. The position of the splines is exactly the same. Also the fact that you had the spacer between you gearbox and bellhousing denotes that you have the long input shaft.
However, having not had the input shafts out, I can't comment on the thickness (and hence strength) of the two shafts, or the teeth on the gear. My guess is they are the same but as the 6 spd takes 250bhp, and the standard 2.8 much less, you need to speak to an expert. Namely, Phil at Road & Race or BGH, whom I believe built the 6 spd in the first place. HTH
Out of interest, what are your main reasons for wanting a 6spd?
Regards,
Giles