There should be a machined pad on the top of the engine block just ahead of the passenger's side cylinder head with a stamped number-letter combination that identifies the year of production and the cubic inch displacement of the engine. The letter suffix behind the numbers is usually the identifier, and there are several websites that list which letter-number combinations apply. The 307 was used mostly in the pickups and other small trucks, not very often in the big ones. A 307 is basically a 283 bore with a 327 stroke engine, designed to give a little more torque than the 3" stroke 283's. They didn't stay in production very long, getting replaced by the smaller-bore, longer stroke 305's.
Jerry
