I could be wrong on the brand of manifold-maybe it was a Howard. I didn't have the actual ad in front of me.
So far I've never seen a currently made 3 single throat set up for GMC sixes. There are several available for Chevies,but none that I know of being made now for the GMC.
Not having heat could be an issue,I hadn't thought of that yet.
Speed

1954 GMC 350
1957 GMC 1/2 ton
1962 Chevy C-30
1952 Chevy 6400 dump bed project truck
'98 Harley FLSTC
'66 Pontiac Catalina
'76 Chevy 1 ton Duallie
'84 Bronco II
'78 Dodge W-200
'81 Toyota 4X4 truck