hi tom
you might have a lens already that will do the trick
do you have a schneider symmar convertible ?
that is a plasmat design, and converted + wide open gives a nice look
( schneider suggests for portraits and landscapes on their website ) other than that
you might keep an eye out for rapid rectalinaers or brass triple convertibles.
rapid rectalinaer lenses are convertible too, so if you find something like a 10" converted it will be longer
and * maybe * the longer focal length you are looking for.
i have 10" and 14" focal lengths that i often use on a 5x7 for portraits, the 14" is perfect for head+shoulders, the 10" is great for wider views ...
often times the older lenses are shot wide open ( which might be too much in some cases ) stopped down a little sometimes
gives a nice soft look without the headachblur that swirly+ubersoft lenses might give wide open.
good luck !
john