I found the answer to my original question...at least as far as running
SWTPC 4K BASIC on an Altair 680. You can do it. The modifications are
described in Kilobaud #8, August 1977. I will put up a copy on my web site
asap.
Every issue of Kilobaud from #7 to #12 in 1977 has an Altair 680 ad on the
back. Byte Mags also have a lot of Altair 680 ads on the back covers in
1976. I think that they were serious about the computer, as an entry level
system. That's just my opinion. I was too young to know first hand.
Bill