What I would like to add to BillT's response is yes the GMC V6's were great for pulling uphills and I've mentioned this before on Jolly's site which is my Uncle drove prototype GMC Tow Trucks that wr given to the California State Automobile Association(AAA) in the mid 50's which were the 55-59 design trucks with the V6 engines. San Francisco was an excellent test bed as it has a lot of hills as well as real steep hills. As far the west coast and central parts of the country being flat I would say that's is really doubtful that let to the demise of the V6, it was all about dollars and cents from the corporate big wigs, It was just cheaper to use a Chevrolet engine
, the GMC V6 was known for it's longevity so it was less prone to break down. GMC historian Don Meyer has even said that. Myself growing up in San Francisco in the late 60's and early 70's and being in a family that owned a grocery market I can say the GMC Trucks powered medium and heavy duty trucks by the V6 were very dominant compared to other trucks
