Well, GMC put a Pontiac engine in their trucks. and Chevy used a Nailhead Buick in 56-59 school bus chassis.
But here are a few non factory swaps:
http://www.spudsgarage.com/vehicles/...PuOR/index.htm
http://cgi.ebay.com/ebaymotors/1957-...-/130770382278
http://www.collectorcarads.com/Chevrolet-3100/49369
Google images can locate more for you.