...instead of adding mounting lugs for different wheelbases. That particular chassis got used in the '67 Color Me Gone Charger as well as the GTO funny cars '67 through '69. They used it for a dune buggy too (!) but that didn't have a body so no rework was needed for the wheelbase for that one.
MPC should have done more of their chassis the way Jo-Han did theirs: same suspension pieces for all, with different length side rails for each wheelbase change. The Jo-Han universal chassis got used in three different length AMC applications (AMX, Javelin/Hornet, Rebel--and there may even be differences among some of those), Olds 4-4-2, B-body Mopars, and Maverick/Comet, all with the same front/rear suspension but different side rails, engine parts, and only a couple of different interior tins and floorboards. MPC on the other hand kept adding on to existing parts most of the time leaving some of them a mess. To be fair, some of their later chassis did use different basic frames with shared suspension bits.
AMT likewise used different side rails, but the Cougar, '70 Mustang, and first version '69 Camaro were all close enough that the same parts could be used in all of them! Only the longnose Mustang differed. The later Pinto/Gremlin/Vega wagon kits again all had a common chassis, wheelbase close enough there./
--Mark