Beehive valve spring must be fitted to the head precisely because that spring is the only one you have and if it fails, it may take the engine with it. Poor fitting valve springs will over-stress the lower coils.
The critical area for fitting beehive springs is the spring pocket. The outer edge of the spring pocket and the valve guide boss locate the valve spring on the head. The locators are very important in order to prevent the spring from oscillating around the spring pocket. That is why it is risky to use "drop in" beehive valve springs. I prefer valve spring pockets that are .010"- .020" larger OD than the valve spring and valve guide boss that is .010" - .020" smaller than the ID of the valve spring. That is difficult to achieve since the valve spring seat and guide cutters may not be available for a perfect fit, depending on which valve spring you use.
On the pair of 1995 L35, 4.3L V6 heads I am having prepared, I am using Comp Cams 26918 beehive valve springs that are 1.310" OD and the spring pockets are going to be machined to 1.350" OD for .020" clearance. Those Comp Cams valve springs are .885" ID and the valve guide boss is going to be machined to .775" OD for .055" clearance. The machinist will use a Comp Cams 4721 seat cutter and a Comp Cams 4726 guide boss cutter. Larger OD valve springs are difficult to fit because of the valve cover supports and head bolt holes. I will use ARP 12-point head bolts because a 6-point socket will not fit between the larger diameter valve springs.
IMO....larger OD valve springs provide more valve spring seat and open pressure. Economy "drop-in" valve springs should never be used with a radical roller tappet camshaft. Roller lifters are heavier than flat tappet lifters and when they are used on a camshaft with more than .480" valve lift, the valve springs should have 330 lb. - 350 lb. open pressure or the lifters may be tossed over the nose of the cam lobes at high RPM and float the valves with disastrous results. The nice thing about roller lifters is that you can safely use a high lift camshaft....providing you have adequate valve spring pressure. High valve spring pressure and hydraulic roller lifters can be a problem with eventual lifter collapse and lifter damage. It would be wise to use Comp Cams (or equal) 15850 short or 875 reduced travel roller lifters. High valve spring pressure also requires full roller rocker arms or you can burn up the rocker arm pivot balls. Full roller rocker arms MUST be used with Comp Cams short and reduced travel lifters. Full roller rockers present stud problems and valve cover clearance problems to solve especially on a 4.3L V6.