The older kits are, very simply, old technology. They were 'good enough' for the era, but you cannot expect those older tools to be updated with better fitting technology.
AMT came a LONG way in the late 90's in The Mueller Era with the 60 Starliner, 70 1/2 Camaro, 58 Edsel, etc. GREAT kits that fit well.
Comparing the old and new kits is not fair.