I have three varieties of tomatoes down in a small garden. I read in
William Woys Weaver's book on heirloom vegetables that Brandywines should
be isolated from other varieties by at least 50 feet, in other that they
"breed true."


That is true of any plant where you wish to save seed. But only the plants from
that seed will affected. If you wish to save seed from any open pollinated
tomato and wish to be sure that the next generation will be true then isolation
is desirable, Tomatoes are self fertile so many seed savers with small gardens
just bag the flowers to keep alien pollen from getting in. A bit of labor but
it works fine.