Slant Six 225 Exhaust manifold question - need help

All slant-6 exhaust manifolds fit all slant-6 engines. No adaptation or adjustment is required. You needn't replace intake and exhaust at the same time, and in fact since the intake-to-exhaust gasket is a common leak point and the underside of the intake "hot spot" (where it bolts to the exhaust) should be cleaned of carbon, you may as well take them apart. However, if you don't want to, there's no difficulty interchanging intake manifolds between the '64 and the '68, either. May as well make sure the exhaust manifold you intend to swap on is worth using -- has a working heat control valve, etc.; see here.

It is very much worth your while to get the good gaskets for the manifolds-to-head and intake-to-exhaust junctions.