This is not the only problem with the Vortec, though a big one. The other problem where lift is concerned is the clearance between the bottom of the retainer and the top of the valve guide, the space occupied by the seal notwithstanding. For a the 062 type production Vortec head, a collision between the retainer and the guide occurs around .43 inch sometimes to as much as .47 inch of lift at the valve. So most everybody just uses .45 inch as a rule of thumb, though that can get you into trouble if you don't check. The answer to this is to machine the top of the guide to provide additional clearance for the retainer. There are also thinner than OEM retainers sold to provide a bit more space. There is also what's known as the ghetto-grind where the OEM retainer and the top of the guide is attacked with a grinder.
So you will not only have to insure the spring has coil to coil clearance usually no less than .050 inch per coil when the valve is fully open. But you've got to get inside the spring and measure the clearance between the top of the guide and bottom of the retainer with the seal installed. If using a positive seal this needs to become a measure from the top of the seal to the bottom of the retainer, this also needs to be no less than .050 inch.
Additionally when getting into really high lifts getting clearance between the side of the spring retainer adjacent to the rocker arm can be a problem. The need here is to grind on the rocker for clearance. Given this is an area of the rocker that sees the most stress, subtracting material can cause the rocker to break with easily imagined consequences. The best solution here and also one that helps these other clearance problems is the beehive spring which tapers in at the top and uses a smaller diameter retainer, thus the rocker doesn't need to be compromised.
With engine building, remember that the cheap way out of problems leads to expensive disasters.