How to id the engine?

Hello all, I just took possession of a 79 f150 and I am trying to figure out the engine size, I was told that it has a big block 400 in it but the motor has been replaced so I don't necessarily believe that that is true. I am wondering if there is a way to id the block some how? it has the distributor on the front of the engine if that helps.
thanks

8 valve cover bolts narrow it to a 351C, 351M, or the 400. The only 100% positive way to tell if you have a 400 is to drop the oil pan and check the casting numbers on the crank. They should be on the side of the first counterweight. A 400 crank will have 5M, 5MA or 5MAB casting code. you may need to clean the crank with a tooth brush to find it. I hope this info will help you