When I purchased my first 5 string violin, an instrument of "exceptionally good" quality made by Berkeley Instruments, on a very popular auction site for a winning bid of $9.00 (shipping was $40.00), I was disappointed at how close the strings were to each other. I am not a professional luthier, but can do some repairs, if the need arises. To correct the problem I first removed the cheaper fingerboard and replaced it with an quality ebony one. Before I attached the fingerboard, I sawed approx. 5 mm off of the fingerboard at the nut end. Then I used the small piece of ebony that was left to fashion a new nut to accomodate 5 strings comfortably. In any case, this alteration gave me a fingerboard width of exactly 1 inch (at the nut). Obviously, it shorten the fingerboard, but widened it at the same time. Now it has a "baroque" style fingerboard of a lengthe of 250mm (but no wedge underneath, as this is unnecessary due to the modern back angle of the neck.)This made the ability to play double-stops much more effective, and I have rather thin fingers, at that.....
Of course, I would not ever condone doing this to a fine instrument, but the price I paid made the experiment viable. Always allow a qualified luthier to conduct repairs on fine and heirloom instruments. One could, if so inclined place a genuine baroque styled fingerboard in place in a modern one and this would seemingly work. Another solution would be to utilize a viola fingerboard if one really needs to have a full length fingerboard. I hope this comment was helpful...