sometimes a blown headgasket wont show up milky in oil right away, let the engine warm up to operating temp and watch both the guages and the engine to see if its leeking anything or trying to overheat.

the chattering could be the valves . have you abused this car?

if you had to add a butt load of oil to get it to read on the dipstick, makes me wonder why it was that low on oil that it took more than 2 quarts to fill it up. never let an engine get below 1qt of oil, you will starve the cam and valve components, and if this engine is a 1st gen small block chavy engine you could have also damaged the piston wristpins on numbers 3 and 4 cylinder. in my experiences with bad oiling on these engines the 3,4 cylinders are the first to start to rattle. but mostly the number 3.

pray and hope this isnt the case with you. or the engine will need a rebuild.

ever read the warning that is stamped on the dip stick?