For starters, if you're not running a game, you should probably consider using vDOS instead.

As for your question, you can mount another directory on another drive letter when you start DOSBox, that is, at the same time you mount the directory where your program is stored. You cannot mount the other directory while your program is running.

Thanks Jorpho
Now got the data loading in DOSBox but having a little difficulty getting the results to screen properly, possibly an inconsistency between old EVA screen graphics capability of the program and current screen graphics. Will try a lot of "what ifs" to see if I can work it out.
The vDOS program seems to be for Word Perfect or a discrete set of preloaded programs. Was a dab hand with word prefect in my youth but now this is far behind me. But will work on it to see if a step has been missed in loading this application.
Thanks once again
denisp

Good News!! Got the screen to work and also altered the config file to start DOSBox and directly load the program I will be using and adjusted the screen to full size. Only one thing left to do and that is to get the screen format to original height and width instead of the wide screen width of my display. Must also add a line in the config file to clear the screen and exit back to windows when I close the program I am running.
All looking good, really pleased.