Bill -- as much as possible, take voltage readings as early as possible -- even while the unit is warming up -- this to check any potential problems before they can do any damage, or make any required adjustments in real time. Particularly the cathode current readings of the output tubes......
Your voltage reading before the rectifier is correct -- as with numbers on the SA-300 schematic, the numbers here are transposed, with the likely intention of the indication being "42 VAC".
The SA-100 will operate just fine with 6BQ5 tubes for testing, or for long term operation. Fisher always specified the latest production tube for a given product (even if it wasn't generally even available yet) -- this in keeping with their desire to keep their image always one step ahead of the game, or as the first to use the newest and latest, and therefore, greatest. At the time, that was the 7189 -- silk screened on even the chassis of the X-101ST before it was even in the RCA tube manual (so was the 7025). In reality however, either of these units will operate just fine with the 6BQ5, with no detrimental effects to the tubes at all.
Good luck with it!
Dave