In large both the 273 and 318 look the same and most everything for a 273 works on a 318 and inversely. About the only thing different are the pistons themself.
I'd stay with the 318
Alan
Heater Control Valve
Factory Part Number 3579173
NORS 1971-73 CHRYSLER DODGE PLYMOUTH FULL SIZE W/O AC HEATER CONTROL VALVE | eBay
Replacement MOPAR 3579173 DT 72-79, 80 TO ½/80, ALL VAN, WGN Valve Heater Water | eBay
There may be others.
Alan
Interesting, I could find no other reference to that. Did find multiple references to a truck ban.
https://www.motorcyclistonline.com/news/road-safety-big-rig-trucks-banned-tail-dragon/
Alan