Hello Clive, nice to see a trader who is enthusiastic about these cars and thinks they're worthwhile refurbishing!
With reference to fitting a 156 V6 gearbox to a GTV, I don't foresee any problems there; the gear ratios are different of course, but in reality this is of little consequence.
The P2 5 and 6 speed gearboxes are identical to one another except that the 6-speed has a slightly longer end casing to accommodate the additional gear. Converting a P2 5-speed to 6-speed is therefore straightforward, as the 'boxes are completely interchageable.
The P1 5-speed gearbox is entirely different; it more closely resembles that used in the 24V 164. Yes, it uses cables (as opposed to the 164's rod mechanism), but they are diferent to the P2, as is the reverse interlock system, and consequently the gearlever assembly is different too (P1 actually has a third smaller cable for the lift-up reverse, attached directly to the gearbox). The P1 has a speedo sender located on top of the box, driven from a gear attached to the diff - the P2 has none of this, taking it's signal from the ABS system. I can't remember what the exact issue was with the clutch (I did this conversion a good few years ago now to a LHD P1 24V), but it ended up being a mix'n'match of components. All in all, the job ended up being a lot more involved than I had anticipated.
Hope that helps!