MMMM, I love sweptlines, haha. Yes, the 426W was optional, it was called the HPP (High Performance Package) and is Super rare, very few in existance. Get in touch with "CSS-Registry" on
www.sweptline.org His name is Ben, and is very knowledgeable about these trucks. He owns an original HPP truck. Worst case scenario, its not an HPP. I dont know if there were any 3/4 ton HPP trucks. The few I know of are 1/2 tons. Is it a "pie plate" truck like mine, or a "four eyed" one, IE, 4 headlights?
Rust was not kind to these trucks, most have rotted to oblivion.
Oh and btw, mine looks much better than that now, and its not even together.
That old 318 in that truck sure took a beating for the 6 months I had it in there, never let me down.