Classic/Antique Car Repair: Firing Order, ford f 100, ford guy

Question
I bought a 1976 Ford F-100, it has an 8 cyllinder engine, but I do not know how to tell what size the engine is.  I do not know if the engine is original, but after changing the plugs it acts as if the firing order is off.  I have asked the Ford dealer and Autozone, they both gave me different firing orders and neither work.  How do I tell the size of an engine,are there numbers on the engine that will help me determine the size.  The Ford guy says it is a 302, the Autozone guy says it is a 360.  Thanks for any help you can give me.  I am happy to send you a digital picture of the engine, if need be.  Thanks again

Answer
yes, there are serial/model numbers on the block itself located either in flat plates design for it or in various regions. It is different with each engine, but the number are there..

Ford can tell you whre to look and what the numbers tranlate into.