As I've said the bike is early 1970s so has 120mm rear spacing and a 5 speed freewheel. Pretty much standard for the day but 126mm and six speed was certainly available. 126mm hubs are listed in the Campagnolo catalogue by request.
I do have a 134mm long Campagnolo record axle and the correct spacers to convert to 126mm and six speed and I will be rebuilding the wheels as the old spokes were too rusty to be useable. I've also got a Regina GS Oro six speed freewheel so I can keep the bike period correct.
I like the idea of going to 126 mm for a few reasons. In particular it means I can use lots of other wheels that I already have. I currently have no other bikes with 120mm rear spacing. The frame needs repainting and a bit of work so I can have it re-tracked at the same time.
The other option is to go for 126 mm spacing and keep the 5 speed freewheel. I can then use the Campagnolo Portacatena that I haven't yet found a use for (though I know this wasn't available in 1974).
What is the considered opinion of the gurus on here?