George is correct, Chevrolet used the 322 Buick Nail Head engine in the 1956/57 9000 and 10000 Series trucks. This was the only engine available in these trucks in both 2 and 4 barrel configurations. In 1958 the 348 became the new engine for the big trucks with the 322 still being used on the School Bus Chassis only, it seems school systems didn't want to be the guinea pig for new engines, so the tried and true 322 stayed on for a couple more years just for them.
The Chevrolet version of the Buick 322 was made with a few upgrades to make it stand up to the challenges of heavy truck use. The water pump was made with a longer shaft housing to accommodate the roller bearing and the timing chain was a double roller (cars used a single roller). It's also rumored the block had a higher Nichol content, but I've never been able to confirm that.
Mike B
