I'd do the primer and first coat before installation and the final coat after. Beadboard will expand and contract with seasonal changes in humidity so if you prefinish you won't have bare wood or primer exposed when the boards shrink in winter. The final topcoat post-installation will hide any scuffs etc that you might introduce while putting it up.

I really didn't realize what a big job this would turn into, but I will follow your advice .. sounds like it is going to take a lot of time to finish. I appreciate your advice very much and will do as you suggest !! (gee, I don't suppose the lumber yard would let me return it all.)