multiple questions

ide be wiling to bet the seat chrome is the same,,,if u have pics of the dart chrome post them,,

the seat foam,,,, i have seen this stuff reproduced,,,,when i asked about it,,they said you wil need to trim it for a final fit,,,legendary,,

as stated the metal dash will interchange to a 65 from a 66 several on this board have done it,, if using the stock 66 gauges,,,im sure you will have to get a 66 dash wiring harness,,,, if going after market gauges you wil make your own wiring not hard,,,
The problem with using the '66 dash harness is that the engine/forward harnesses are different too. You'll have to splice somewhere no matter how you look at it. If you use the '66 dash harness and fit it into the '65 bulkhead connector, that would probably be the cleanest job but there are still some wires that are different.