It would depend on the system software in particular, and possibly the monitor also. There is occasionally a need for considerable fiddling over this, for some reason, so if you don't succeed, report back.
If you have the Monitors and Sounds control panel included in the AppleVision 1.5.3 and Apple Displays 1.5.5 software, or versions of the system from (as I recall off the top of my head) 7.5.5 or so, then you should be able to adjust the screen geometry through the control panel. I do not know if this works with, say, an LCD running from the machine through an adapter, but it certainly works with the Apple gear of the time.
I do not think it was possible to do this in the version of 7.1 Pro that shipped with the 840av, but the relevant software upgrade can be
downloaded from Apple's servers. Pay attention to the txt files associated with the two packages.