Shortening Up Rear Hatches on a '74 20' SF

[pluffmud]

Got two rear hatches off a '73 or '75 for my '74 that are a couple inches too wide. The hatches appear to fit snugly with the existing channels that are in my boat.

What is the best way to go about cutting out a middle section of the two hatches and joining the two pieces back together?

Take out the core on the underside, make two cuts on the hatch, grind the underside and apply 1708 and CSM. Then put CSM on the topside and recore the underside of the hatch? Or would it just be best to use the hatches as templates and completely redo them.