Heads.

OK I did get a cam yesterday,so now I am on to the heads,i know i will need new springs comp 901,but whats up with the exhaust vale springs.Why is it different.I know the comp springs will work,but do in need new retainers on the exhaust side? also these are smog heads, some 340-360 are some what the same,should i look for 340-360 that are not smog.