Whoa. You said you used the stock lifters?!?! You should never ever use a flat tappet cam with used lifters. The old lifters are married to the old lobes of the old cam. I think it is safe to say that you probably have irreversibly damaged your cam by reusing the lifters.
But back to DoubleVision's point: I think the easiest thing to do is replace the valve springs. With the correct ones. You can do this with the correct tool without removing the heads. Here are the springs.