Valve seals do not fit on my valves?

ok i had a blown head gasket took them off them then i found out that one had a crak so my uncle had a couple of head on his shop for a chevy and told me to use them so i look to make sure that they were the same they looked the same but when it came to putting the valve seals on the valve it turns out that the valves are a little tiker that the ones from the set i measure the valves and they are 3/8 thick and the seals are 5/16 so i just need to know what to aks for when i go to the auto parts i tryed to use the valves from my old heads but they are very lose on these ones