Go and read some books by David Vizard. Try the Comp Cams and Crane Cams website for some good pushrod engine info.
Pushrods are not
that important if you're trying to figure out where to spend a bit of money on an existing engine for a hop-up. The majority of the mass in an OHV system is in the valve and the valve side of the rocker arm. The bigger the rocker ratio (to get more valve lift from a certain lifter displacement) the worse the problem. However, yes, you want the lightest and stiffest pushrods you can find. I aced a deal on some carbon fibre pushrods for my 327 Corvette engine, but I only fancy testing them on a back-to-back basis with the middle-quality TrickFlow steel items I have.
And light lifters are important... but if you're going mental with a cam lobe to get lift and duration, then you're probably using a roller lifter, which is heavier than a straight 'solid' (heh - actually hollow!) non-hydraulic lifter. (A hydraulic lifter contains oil which varies in quantity, squirting out through orifices in the lifter. The idea is that it removes lash from the system and is quieter. The drawback is that to be quiet, the orifices need to be quite small, and the higher the revs, the oil gets trapped and can't get out, which causes 'lifter pump-up' and makes the valvetrain too tall, holding the valve off the seat when it's off the cam lobe, losing compression - and thus power.)
If you're going above 7000rpm then it may be worth looking into a 'rev kit' which is a set of reaction springs that hold your lifters as gently as possible against the cam, thus reducing the load they transfer back to the spring holding the valve closed and the stresses on the pushrod. The logic follows that a well selected valvetrain for use without a rev kit will be nowhere near optimised for use with a rev kit.
Most important of all in a performance OHV valvetrain (assuming you've got your valvetrain as light as you can) is spring selection. Springs themselves have a weight, and the valvetrain's weight and dynamics will be affected by that weight. Actually, this goes for OHC engines too, but in an OHV engine it's arguably more important because of the increased number of components.
For a given big cam, you might need a high spring rate of X. So, you select a spring of rate X which is a double spring set weighing 120grams. Now, if you choose a slightly lower rate, but is a single spring which weighs, say, 80grams, it might actually allow valve control which is just as good up to 6,500rpm, and then actually more torque above that because the spring is controlling the lower mass of the whole train better - no valve bounce from all that extra spring mass.
It's a heck of a game, cam selection. It's not a black art, but the level of understanding and experience necessary to optimise an engine is huge. Even the cam suppliers will be a little bit conservative when you ask them what cam their experts recommend. If you're going for the last tenth then you have to learn some of the fundamentals yourself and figure out where you're going to take the risks.
You may be interested to know that the camshaft may be finally on its way out. Manufacturers have been experimenting with solenoid operated valves for a while now - infinitely variable timing. However, the solenoids are bulky and current materials technology and heat issues mean that valve control is only good up to around 5,000rpm. However, the engine up to that point makes very impressive amounts of torque relative to current state-of-the-art engines. When materials and solenoid technology improves, the capability of these engines will improve. When they're up to 7,000rpm limits at good endurance, expect to see them in the market, linked up with turbochargers on smaller 'eco' style engines. Ironically, these devices are better packaged inverted and driving rockers in pushrod style V-engines rather than OHC engines, because of their size. You add mechanical complexity, but the engine remains the same overall size. The OHC applications are quite bulky and make an engine topheavy.
J/K on the little man thing.
