Sorry Matt-
If you leave the old lifters and pushrods, you just bolt down the new rockers. Check the geometry. You won't be able to do anything about the lifter height. Shouldn't have to.
New hydraulic lifters: If you take a new lifter out of the box you'll see that you can push down the pushrod seat about .150" or so. It will be pushed back up by a spring. The hole in the side is where oil from the block comes in. There is a ball check type of valve in there that will let oil in to keep that seat from moving up and down (hydraulic). This is what happens when the seat comes back up. If there is any play in the lifter pushrod rocker valve line, it will be taken up to an extent by this. Once the lifter fills, or primes to a height resulting in zero lash, it stays. A common way of installing new lifters was to bleed them of any air (coffee can trick that PKRWUD said) and intall them. Then they will be hard like the ones in your engine now. However, if you do this then you can't measure preload height (distance the pushrod seat travels when you tighten the rocker). But you don't have to shim alot of cars now either. This is because newer hyd. lifters adjust to a height that is zero lash and stay. Old hydraulic lifters could be over primed (too tall), new ones cannot (anti-pump-up). Or so they say. I can atest to running a high pressure oil pump that didn't over prime comp hyd. lifters.
You don't do anything with old lifters. You could pull them, apply assembly lube, and replace them exactly I guess. I'd leave em.
Confused? I think I had too many beers at our Christmas lunch.
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