The definitive work on the 2600 Tire Dilemma can be found on Robert Piacentini's exhaustive page:
Alfa Romeo 2600 Wheel and Tire Talk BTW, also great info on the Weber conversion debate.
At the time of his writing, 16" tires were not readily available. That would be the closest match. By going back and forth between Robert's page and a tire site, like
www.thetirerack.com and comparing overall tire heights (in my opinion, the critical measurement to retain as stock appearance as possible) you should be able to find something reasonably close, though now the challenge becomes finding something skinny like the originals!
As an aside, I spoke with a nationally known racing wheel manufacturer about making some reasonable looking replicas for the 2600. When I showed him the stock rim, he was aghast. The bead sealing area is the problem with these cars; he referred to them as "farm implement" style beads/rims and made it clear in no uncertain terms that they were dangerous. The drop down, modern bead style is the main reason for wanting to make this switch for safety reasons. Alas, it is the drop-down bead which causes the interference with the calipers making retrofits hard to source. You need to have a custom rim made, which is really not all that expensive.
Some will make the argument that they were fine back in the day, even that guys actually raced these heavy cars on skinny tires and survived, but I believe for our purposes we should have the most modern technology available to us, whether its electrical, brakes, fluids or tires.
Having said that, altering the suspension, i.e. spacers, to fit other rims is more dangerous than using the original rims and tires.