55-56 GMC Exhaust
 

Looked at a 55-57 GMC Truck and with that Pontiac V-8 under the hood, I would bet it was a runner in the day. So to those in the know, were headers part of the factory assembly? They look factory and nicely bent tubes for clearance. Never have seen one before. It has a 4sp truck tranny and 2 barrel carb.

The Pontiac V8's used in '55-'59 GMC trucks all used basic cast iron manifolds. They may have been identical to Pontiac pieces, but I'm not sure. They don't look any different than Pontiac, at least from the top- perhaps the outlets were slightly re-arranged.

Apparently you were looking at aftermarket headers that someone added to the truck. Were they 3-tube headers? Center exhaust ports of '55-'58 Pontiac engines were siamesed.

maybe I can get some pics taken and post.