Thank you very much for sharing your experience Tom j and
Duff. It's raining here today, so I am not doing more than a
little measuring, cutting, and gluing. I will try the teflon
piping dope.

Unfortunately the valves have to stay in place, and it's a
bad idea to replace them due to the piping starting
underground within about six inches of the valves yada. So I
can't try adapters and valves at the supply store to get a
good fit in advance.

I will report back after the job. It's a little tricky
because I have to glue other joints before I can apply the
operating pressure to the two adapter-valve joints under
discussion. And as you all know, once glued... I may put in
unions to help ease the pain of a mess-up at the valve(s),
due to yours truly not being experienced enough in plumbing.