All I've done are with solids, after re-reading some of my "sources" you should make the hyd lifter solid. Like Sitdown did. There are some post on the bottom that talk about this.
As far as weaker springs (or checker springs), I don't think a weaker spring is going to change the geometry from a static point. The cam will push on the push rod to the rocker to the valve. I can see while in motion with flex (operational RPMs), and other things occurring how the geometry could change, slightly. The problem with not checking with a weaker spring is you could very well bend the checker pushrod, and your measurement will be off. Ideally, I'd check them the springs you are running, but I bent my first tool doing this.
Centered on the valve with the narrowest band possible is ideal. I'd take an offset narrower band over a wide centered band. I saw this on the corral post below, and then in a couple other post. I thought about it, and from the dynamic loads produced from the rocker going back and forth on the tip I believe the loads on the guides will be higher than a slightly off centered, but 90 degree valve to rocker relationship. While the roller rocker tip will reduce friction, and allow the tip to rake back and forth, this will produce side loads on the valve (which will affect guide wear). I am sure the guide will provide some support to a slightly off centered load. I have not done any math to try to compare the loads, but I can't see the off centered load being greater than a constant side to side load. I am sure there is a limit to how far off center is acceptable, but I am not sure what that would be.
I am not an engine builder, and do not claim to have all the answers. Heck my sources are from folks on the internet who I've never met. I am just trying to look at it from an engineering stand point, so I could be way off.
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Pushrod length - Yellow Bullet Forums
Bad info floating around about measuring Pushrod length - Ford Mustang Forums : Corral.net Mustang Forum