Basically the path of the roller needs to stay in the center meat of the valve stem. If it stays on the edges then it will accelerate valve guide wear. If the tip rolls off the side of the stem...oops, bad news.
Here are some things that will impact the correct pushrod length from one engine to the next, or even during a rebuild...
core shift causing misalignment of the cam shaft bore (or align honing the cam bore)
base diameter of the cam shaft
variation in height of the lifters (brands/designs/models)
pre-load on the lifters
block deck height
head gasket thickness
cylinder head design/casting (including how well the rocker studs are aligned with the lifter bores)
milling the cylinder heads
different brands/models of rocker arms
valve stem height
So I hope you can see the variety of factors involved here and that you just can't blindly swap rollers. Old stamped rockers are not sensitive to pushrod length like rollers are. You must take these things into account.
You can make an adjustable pushrod length checking tool, click on the image for details:
For checking the geometry, here's a short page. The general consensus of engine builders is to shoot for a narrow roller-to-valve-tip pattern, rather than a centered pattern. If you can get BOTH a narrow AND centered pattern, all the better.
Other areas of the valve train that need to be checked are here.
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