had what sounds like the same problem 12 yrs ago.
friend said it is quite common on his low buck rebuilds.
he said just drive it around.
i did, and within 50 miles it broke in (valve seat to valve) 1st 10 miles were awful
i've since seen stubborn guys take off theheads and have them touched up.
the problem was, i bought bare heads, and the pre-installed seats had a slight distortion from being pressed in. being a low buck build, i wasn't about to take them to a shop.