The usual process to consider lower gears starts with the rear cogs. Then goes to the rings if there's not enough change from the rear only.

Questions- What's the current gearing, tooth counts? How much lower do you want to go? Do you wish to keep as much Campy as possible? If so how does this preference balance against your want for lower gears?

How these questions are answered will guide the choices.

I'd suggest that you consider moving away from the Campy you have (at least on the bike, what's in the box??). Campy NR has never shifted as well as more recent designs and has limited gear range, even with the tricks and such that can be played with. If you move to a triple crank then the BB will go too. Even with better shifting gears the racing nature of the frame has it's aspect. Expect some chain rub in near crossover cog combos. The likelihood of Italian BB shell will make the BB replacement a touch more challenging to get. The ft der is likely a double caged version. This will hinder a triple shifting and ring diameter range. The braze on aspect is less likely to be a problem. The 126mm rear end could handle a 6 speed FW or an older cassette hub. This frame could be stretched to a 130mm which would open up a lot of current gearing options.

Figure out what to do with the bike before dealing with the left over parts. Feel free to ask more but we'll need more data points to be more specific. Andy.