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.

~~ Jethro
1954 3100
Back to Life
In the Dity Gallery
1951 3100 (gone) / 1956 4400 (still in the neighborhood) / 1957 6400 with dump body (retired) / 1959 3100 panel (in the woods junked) / 1978 Custom Deluxe K10 / 1993 S-10 4.3 / 2004 Chevy Crew Cab / 1945 John Deere H / 1952 John Deere B / 1966 John Deere 2510 / 1967 John Deere 1020