6 speed hubs are narrower , so frame is narrower too , have to do more to pry the frame wider..
might need hiring local help on that.
8+ speed need 130 wide hubs , 6~7v were 126.

maybe relocate the friction levers to the front of RetroShift brake levers,
if the need to have the shifters off the frame, ( I did that because I got Old.)

I like bar end shifters myself, that's way simpler.

Maybe find more vintage Campag components,
so there doesnt need be the coldsetting, to stuff in wider hub wheels.

or start over with a more modern frame..

only my 3 speed has a click shifter..