Replaced both rear floor sections, right crossmember & both inner firewall
sections in Nov. I am told there is a production problem with left floor crossmembers.
Is anyone else having problems acquiring this part? Other than that, the fit & finish
of the sheet metal is excellent.

I assuem you are getting this from Restoration Design? There are a few other vendors that may stock some of thier parts, or OEM. BUt I hate to say it the sources for these parts are very limited. You may be able to find a suitable used one, check the classifieds. Rhodyguy does a very good job of cutting these so you can use the part you need. That's probably your best option if these are going to be awhile.
-Mark