I have recently switched engines in my 78 F250 that has a 351m in it. My old engine always took 6 Qts oil after an oil & filter change. After switching engines 6 Qts. only gets it to the add mark. I used the oil pan, dipstick and tube from the replacement engine. Did any of the 400's or 351m's have a higher than 6 Qt. capacity oil pan?

 

My 79 F250 with a 400 takes 7qts, sounds like what you have now.

 

Ok; I didn't know that there were 7 qt. engines out there. I take it they use a different pick up tube! I'm glad I used the one that came with the engine and not my old one.