that comes into play when you go with solid lift cams because the lifter dont give you have to run clearence in the valvetrain the valve lash you refer to is how much clearence is between the rocker arm and the top of the valve.
it is measured with feeler gauges and you must have adjustable rocker arms or adjustable pushrods to adjust the clearence..
Adjustable Rocker Arms eh.... Well, since ours are shaft mount, there's not a whole lot of adjustin to be had with them...of course, if you need that much, you have problems already. It comes into play with solid lift cams cause time was, you had to yank the valve covers every few thousand miles(and I don't mean every 10k....more like every 1500-2k) and readjust the valve lash. I was taught to do it just by felling tension on the pushrods....this was on a SBC where clearances are a bit more forgiving in stock applications though. When you felt the pushrod clamped and it wouldn't turn easily, that's when you torqued it(I believe it was about a 3/4 turn or so). The engine I did this on is still running today(actually, it's the first engine I ever rebuilt...305 though. Not much to be proud of). I guess he's right, it would be easiest to get some adjustable pushrods, although, it's not absolutely necessary,..in my opinion.... Though, I could be wrong.....but I doubt it He ius right though. It's the clearance between the valve stem and the rocker.