this may sound stupid but how would i find out what valve seals to use? the heads are 6272990 rectangular port bbc. i'll be using double valve springs witha dampner. can i use some from napa that were on a stock head as these are a GM head?

You will need to take a spring off and measure the size of the guide. The stock guides are .625 od but they are pressed in the heads blank (with no center hole ) and drilled when the seats are cut . The inside of the guide is not concentric to the outside. So , the reason why im telling you this is because if you put a posi type seal on the guide without machining it down to a smaller size, it may not seal because of the id not being centered. Popular od sizes are .562 and .530 for a guide that has a 3/8 valve stem.I can hook you up on some seals if you have troubles locally, Dennis