3.5" drive on PC6300???

[bchs1b at elroy.uh.edu]

I wish to install a 3.5" floppy drive in my old computer at home which is
an ATT PC6300 vintage about 1984. It came as a 2 floppy system and I have
since added a hard drive with its own controller.
Will I need to do anything special to run a 3.5 drive other than upgrading
to DOS 3.3? Do I have a choice of using a 1.44 drive or will I be limited
to 720K drive. Will my floppy controller run both fine, do I need a new
controller for either? Any information appreciated before I forge ahead. Since
I presume I will have to lose my 5.25 drive to install an internal 3.5 would
it be much more expensive to install an external one?

thanks, 
mike benedik
benedik at uh.edu