Here's a little info not all what you asked for but a start.
The 426 GMC inline 6 appeared in 1940s 4x4 Cab Over Engine (COE) trucks made in Pontiac, MI. It also appeared in large GMC trucks in the 1950s.
[edit] 503
The 503 GMC inline 6 was more numerous than the 426 inline 6, but beginning and ending dates are unknown. The GMC 630 Series of the early 1950s offered the 503ci inline 6 engine.
