Cam selection help please

You're pretty funny StrokerScamp. Sometimes I just need things spelled out for me to see the light! Remember I am an ex-Chevy guy right?! So I guess you have sold me on this XE275 HL cam. 1st questio: Do you think I can keep the springs that are already in the heads? (previous owner states they are single springs and seat pressure of those is 300 lbs). The ones that I saw Comp recommends are dual springs at 350lb. I may have answered my own question, but then I guess I'll have to have the heads machined to accept the dual springs? 2nd question: What kind of gains do you think I will see switiching from what I have now to this new cam (7 degrees greater duration on intake and exhaust and .045 greater lift) from what i have now? Thanks