Dodge Dart Alargado (limo)

cudamark said
I wonder if it's a late '66

This kind of Dart was offered new in Spain from '66-'69.

and/or started earlier than the U.S with the dual braking system

No, later.

and the Midland type booster

Almost certainly a local Spanish booster.

I see a relay setup on the inner fender, maybe a Leece-Neville alternator?

Local Spanish alternator. The relays you're seeing were added long after the car was built.

and what are those orange items on the firewall?

From here they look like build-era relays (like starter relays).