The metal frame of the dash is different. Besides the ignition being in the dash on the 68, the pads hold on different. Also
the dash wiring harness is different. Have to change the steering column too. Still more on the dash you will need all the panles not just the cluster; radio plate, piece above glovebox and ends also.

And when you say rear tail lights realize its the whole back panel, not just lights. With the grille the whole headlight openeing system is different too.

Oh and dont forget the gas cap

Personally I would think it would be cheaper and easier to start with a 68 instead of killing a 70. Unless all those parts are bad on the 70 to begin with. But hey I've biased towards 70's.

But.......... if you decide to do it I could possibly use the fenders if in nice shape.