I think you should first try keeping the original campy derailler, install as large of a rear cog as you can get to fit with the campy and see if that is sufficient. A 42x30 is a pretty low gear and you can avoid the mega-jump between a 22 to 34 tooth gear to still have a nice progression of gears available for climbing. If you put a cheap shimano long cage derailler on this bike it will adversly impact the vintage character of the bike.
For a bit lower gearing, I would think that you might be able to modify a cheap and easily available 110mm BCD chainring to work on the 116 crank by ovalizing the chainring mounting holes outward my 3mm each.