I would use two tools for this: a thicknesser and a radial arm saw. Put the old chopping boards through a thicknesser until both top and bottom are clean (smooth)and parallel - that's your jointing done. Get a radial arm saw set up as near to perfect as you can, clamp a stop to the fence that will determine the future thickness of the board. You can just feed the old board in from one side and cut strips off which you can turn through 90 degrees and glue back together.
If you haven't the tools, try a local joiner or the school woodwork shop (nightclasses?). This is by far the best, as well as easiest method i.m.o.
-P.