The TR4-6 valve should work as a replacement; although you'll probably need the 45 degree street ell to connect it to the head, and do something creative with the water line.
Another thought might be keep the TR3 valve under the hood; and add an in-line valve where the hose comes through the dash (on either side). Lots of Chrysler products in the 60s and 70s used in-line valves with 1/2" hose barbs. Try a 65 Dodge Dart.