Just wanted to add some current info on the vacuum advance can for the 480 distributor. I am almost done rebuilding the 302 for my '69 Z and after some research realized that this was an important part of my engine assembly. Even though my car ran fairly well previously, I tested the existing B28 vacuum can and it did not function properly (per the specs provided via this forum - Thanks!) and wouldn't hold advance with vacuum applied.
So off to the parts stores I go with my parts number list courtesy of CRG. Most stores (O'Reillys, PepBoys, Napa, Autozone and a couple small stores) did not have the listed part numbers, but were nice enough to order me an advance based on the application (the only way they really know how to look up parts on their computer nowadays). Man I miss the old days with parts books and interchanges and spec books. Of course each one of those were incorrect, with some being B1's, B25's and one with no number stamped in it at all.
While I was at Autozone examining yet another incorrect part, I asked them to see if there was an "1810" in their brand parts list (Duralast) and there was - a "DV1810". Of course they had no specs on this item but I ordered one anyway - at $11.99. It came in today and low and behold, it has B28 stamped in it. Eureka. Oddly, it didn't even come in a box - only a FedEx bag.
During this adventure I checked some of my other Z distributor vacuum advance units. Turns out there was a B1, a B20, and another bad B28, so I ordered three more today when I picked up the first B28. Sure hope they are the same upon arrival.
Once my distributor is set up on the Sun Machine, I'll try to post the resulting performance (vacuum and mechanical advance stats).
Final comment: If you think you may need one in the future, it might be wise to go order a couple while they are still available. Who knows when the supply could dry up....