I'm not so sure that the 288 is a Pontiac motor. I am sure it's a motor that GMC used.
The 288 V8 engine was used for a short period in the beginning of the "Y" series GMC trucks. "Y" series trucks are so called "1955 2nd series". In the latter of part of 1955 and all of 1956, GMC went to the 316 V8.
