Campy didn't make a 39 chainring to fit that crankset, although there might be an aftermarket one that will fit the BCD. While an earlier response suggested that you will need to change the NR rear derailleur with a wide range freewheel, you certainly will not have trouble with a 26 tooth big cog, and you may do ok with a 28. Trading the NR for an SR rear will definitely get you to the capability of a 28. Another (albeit more expensive) period correct alternative is a Record triple, but they are not very common and will probably require a new bottom bracket as well (different axle lengths). Battaglin used one in the mountain stages of the Giro in the early 80's.