AD trucks had no heater upon delivery. It was a dealer-installed option, so the dealer installed the heater of your choice using standard jobber hoses. Many chose the Chevrolet or GMC marked Harrison fresh air or recirculating heaters, but many folks installed other brands. My 51 3100 had an Arvin heater as well a my 57 6400 and 59 Panel. My current ride, a 54 3100 that my dad bought new has had a Mopar junkyard special since it was new. All are correct since none were offered "stock from the factory."
So... the answer is... go to your flaps ( friendly local auto parts store) and get black or red heater hose. Either color would be period correct.
Unless it's a concours restoration, then all bets are off.
