Have had numerous issues with valve train on older 396. Originally had low oil flow to top end, had camshaft grooved at rear bearing, massive oil flow now. Put motor back together ran on hose for 20 minutes. Took boat to the lake ran for approx. 2 minutes and started to run rough, found 1 broken intake pushrod and all other intake pushrods bent, exhaust are all fine.
Intakes valves chase the piston down the hole. Pistons chase exhaust valves back to their seats. Much easier to smack a exhaust. Float em, and you find out. Nobody ever hit a floating intake. You are in coil bind. Cam most likely has slightly more lift on the intake side. If you are in coil bind(stock springs?) you need to replace them all. Assume the cam isn't stock, due to the fact that it wasn't grooved to begin with. Stock BBC springs, specially early ones, don't have much in the way of valve spring when it comes to lift.
Did not take long to figure out what happened. I had purchased a new timing chain and gears that had multiple marks on it, i had the valve timing off. Needless to say the intake vales all paid a visit to the top of the pistons. Pistons are all ok, took the heads to the machine shop for new intake valves.....Hopefully this third time will be a charm....I am wearing my motor mount bolts out.
I just had a customer's engine this week that was built by his local machine shop that had the pistons put in backwards. Waiting for him to get them turned around and bring it back so we can proceed. Fairly common mistake.