If you go ahead with Casey's recommendation (a good one, by the way) just be careful to gauge the alignment of the grove to accept the spline from the rear of each board since they're probably not all the same thickness. If you measure from the front on some and the rear on others, you'll find that they won;t line up very well when you go to put them back on the wall. By measuring all from the rear, you'll be assured that the rear of the planks wil all be in the same plane, making it easier to re-attach to the wall, assuming that the wall is relatively flat.
Another potential solution would be to use a very high elasticity caulk in the gaps. I've had luck filling fairly large gaps using a constuction caulk/adhesive called SIKAFLEX. I get mine at Home Depot, usually in the MASONARY aisle. I guess the company mostly makes masonary caulks.
I've used this successfully in gaps up to a half inch wide next to 18"+ wde boards. the caulk compresses and streches beautifully in response to seasonal movement of the planks adacent to the gap.
I installed the caulk in mid spring and mid autumn in an attempt to place it at the midpoint of board movement.
It is advertised as a sandable caulk, but in reality, it's too pliant to sand well, so you have to get as good a finish off the gun or putty knife as you can.
The lord hates a coward....