The pipe would be too far off the wall, and the coupler thread is not very good and not even round at the exposed end

Anyway things have moved on - managed to escape for an hour this afternoon (guests running late

) so removed the lathe tail stock and mounted up the pipe, coupler in the chuck.
Initially I turned the pipe up to the shoulder down to the minor thread diameter, then about 50 thou below, and even tried to undercut the shoulder but it wasn't having it, the weld penetration must have been good!
So I decided to part it off, and see if the remaining thread would work with the elbow (It's a BSPT thread so tapered)
Sadly not - I can get about half a turn on. So I decided to mount it up and re-thread it on the lathe. Carriage to extreme right which involved removing the taper turner bracket and RH stop mechanism but still I couldn't get the threading tool far enough to the right.
Now I must study pictures, but I think the I can shorten the pipe at least 1" maybe 1.5" which will solve the problem, but time caught up with me and I had to go 'meeting and greeting' !
. . . .so hopefully a job for tomorrow