I have a '73 F350 with a 390. Last year we replaced a bent push rod. All was fine and dandy until last week. We took the truck for a good highway run. The truck was hard to start and it back fired. After that it was making a clanging noise. No big deal I thought, sounded like another bent rod. Pulled the passenger side valve cover off and found 4 bent push rods. Bought some new rods and all but 2 went in OK. For 2 of the rods, it feels like the valve lifter is missing. There is nothing stopping the rods from disappearing in to the engine. Is it possible that the lifters came out of their holes? Eventually I'll get around to pulling off the intake and look.

 

I would bet the lifters popped out of the holes. You're going to have to pull the intake and hunt them down. That's the bad news....
Now for the even worse news. Lifters and cam lobes mate for life. Getting the lifters back in the wrong locations will most likely result in having the lobes on the cam wiped out.....

 

How's your oil pressure? If the lifter did come out of it's bore you'll have a huge internal oil leak. I wouldn't run it untill you've checked.