Check out Jolly's 6066 GMC website regarding GMC engines!
Here is the GMC inline 6 information.
http://www.6066gmctrucks.org
In 1964 GMC offered the Chevrolet 230 inline six engine, it brought economy, efficiency and low initial cast to the line duty trucking industry. The time-proved 6-cylinder engine is a product of GMC Truck and Coach Division's 50 years of truck engine design experience combined with the vast resources of General Motors Research and Testing Laboratories. These engines are the same used in Chevrolet Trucks, and GMC Handi-vans. Also these are the same engine used in Canadian Chevy and GMC trucks.
GMC (Chevrolet) 250 Inline Six
The Chevrolet 250 L6 engines also offered in GMC Trucks, this one in 1966 it both the States and Canada. Here is a list of Inline Six Cylinder Engines used in GMC Trucks in the 1960s: 194 cid, 235 cid, 250 cid, 261 cid, 270 cid, 302 cid. There was also a 153 cid inline four used in the Handi-Van.