In '55, they did two different body style trucks. Thsi was mainly due to the push to get the new body passenger car in production, the trucks had to wait. So until around March of '55, the trucks were basically warmed over '54s, with minor trim changes, and on 1/2 tons, an open driveline. The engines were carryover from '54 as well, with the high mount water pump. Corvettes were also carried over, with the first 6-12 (sorces vary) having the 235 and 6 volt electrics.
The advantage with the 261 comes down to it being simply a larger engine. Bigger displacement, more power potential. They were mostly used in 2 ton and larger trucks, plus the 2 ton based busses, it was also used on lighter trucks by the early '60s as well as being used in Canadian Pontiacs. The last US use I'm aware of was the '63 3/4 ton 4x4 Chevrolet pickup.
