Roger, the objective is to have the rocker arms move in a range that works considering their fulcrum location and ratio, and one of the biggest reasons that longer pushrods are recommended is because of the shape and size of the cam lobes on a high-lift unit. The stock cams have a big base circle, and the cam bearings are only a given diameter. The high-lift cam uses a smaller base circle and still has to fit through the same hole in the block so can't be just ground with higher lobes.
If you consider the math for a new cam with about 0.500" valve lift, they grind the base circle a little less than 0.050" lower than a stocker. But the valve is going to open about 0.070" more than stock (remember the rocker ratio). So what do you do with the rocker position? If you use stock pushrods, then you lowered the pushrod end 0.05" and you will operate the valve to 0.02" beyond where it used to go. If you use roller rockers, they sometimes have a different fulcrum position, and/or use 1.70 ratio instead of 1.75. So it's a balancing act to find the right pushrod length so the rocker is in the right position to operate best.
Most competent hi-po engine shops have checking equipment to determine what length to use.
I will agree that Leo made a good recommendation for most high-lift cams, and will add that the NAPA pushrods will probably work fine, since I think you inferred you are primarily after the harder material. The extra length would be nice, but it isn't absolutely necessary.
If you have stock-length valves installed in your head, I would recommend that you do NOT use the Crane pushrods. They will work, but are designed for longer valves, and take the rocker geometry to the other side of where you are now.
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David
newbie #4153
