I'm looking for a parts (car) for my 1963 Plymouth. Mainly front clip (inner fenders & radiator support), firewall, upper cowl, dash and everything under it, manual pedals. Finding a decent '63 seems to be hard, but some good '62's and '64-65's pop out every now and then. What would be "most correct" replacement donor for the '63 for the parts mentioned above? I know they propably will all "fit", but I'm after a "factory" look. Thanks.

For sheet metal and interior parts 62-63 will be identical for mechanical like pedals it could go through 65.

I think you can use the fenders and doors. I beleve they will fit but not sure if you can use 63 doors with 62 fenders or not.

Yep, big difference externally between the '62-3 and the '64-5 bodies.

I used to have a cool cutaway that detailed the differences (especially in the roof height) but it got lost when I swapped out computers.

'62-'63 are most similar; cowl/dash/windshield area changed for the '64s, front upper door/vent glass frames changed too.
Firewall area (under upper cowl panel) looks very similar between my '63 & my '64, aprons, floors & some structural members are probably interchangeable. Lots of 1-year-only things on the '62s IIRC, mainly window mechanisms, rear doors, exterior sheetmetal. '62 - '63 dashes pretty much interchange, though the clusters are year-specific.
Some day I hope to have a nice '62 Savoy 2-door, always liked them.

IIRC the front bumper is the same for '62 Dodge and '63 Dodge as well.

The doors will not interchange unless you use the same year front fender. The big body "bump" on the '62 Dodge is the problem.

And the 62/63 use different latches and striker plates than 64/65.