my 59 gmc has a ponco motor in it from the factory. 55-59 trucks had a version of the pontiac engines. my 336 is based on the 389. It has the same stroke but smaller pistons.
From the web
"Prior to developing their own engines, GMC used the Chevrolet Straight-6 engine and Pontiac V8 engine. They used the Pontiac 287-cubic-inch (4.7 L) motor for 1955 and 316-cubic-inch (5.2 L) motor in 1956, but advertised the engines as the "GMC 288" and "GMC 316". They used Pontiac's 347-cubic-inch (5.7 L) in 1957. For 1958 and 1959, GMC reduced the bore of Pontiac's 370-cubic-inch (6.1 L) to 3.875 in (98.4 mm), resulting in a displacement of 336-cubic-inch (5.5 L). In Canada, however, GMC used the Chevrolet Small-Block engine rather than the Pontiac."
