I finally got around to swapping my iron heads (set up with ductile iron rockers and the original stamped steel hold-down tabs).
Now the aluminum heads are going together on the bench, and of course the valve gear doesn't line up the same with studs and aluminum hold-downs.
My cam has .652 gross lift and .018 lash, with spring pressures of approximately 130/430 lbs.

How critical is lateral alignment with the iron rockers? They have a pad that slides across the valve tip, not a roller... the old valves show a "stripe" that goes all the way across the tip, and actually at an angle!
I figure they can slide in any direction as long as there's enough oil getting to the rockers, and there will be little additional loading on the guide for the same reason.

Thoughts?

My thinking on this is that the loads are already offset due to the pad and pushrod positions. So as long as the pad makes full contact across the valve tip that should be fine. Centering is not critical.

Are you using springs or spacers? I ran Crane ductile iron rockers that were bushed by Rocker Arms Unlimited with a 0.590” solid roller. They supplied me with spacers and shims. I apologize I don’t know the exact number I used for side clearance (I think it ranged from 0.015” to 0.025” per pair). I also had a wear line on the valve tips, but parallel with the rocker shaft like you’d expect

I'm using the original rocker arm springs that came with the set.

After measuring, if I take .080" off each side of the aluminum hold-downs, the pad will be centered over the valve tip, and only thin shims to provide something for the iron rocker to ride against other than the aluminum.
IF they fit under the MP valve covers that is. Otherwise it's back to the stamped steel (stock) ones!

Incidentally, my cam is a slightly reground (to restore lobe taper) Mini-Express with mushroom tappets. It was 278/.654, now 272/.652 with less (.018) lash.
I bought the rockers from AndyF at his recommendation - valve springs are Comp 953 with 130 seat, 430 open as previously mentioned - so I don't need high-dollar rockers. Yet

Who re-ground the cam ?

Schneider Racing Cams. They also refaced my mushroom lifters The (reground) lobe is now stored in their library if someone else wants one.
I have a spare set (new-in-box) too, in case I feel like running another mushroom cam someday!

I would ditch the springs and put alum spacers between the rockers with a thin steel shim between all spacers/holdowns and the rockers, IMO the rockers will move more smoothly without the side pressure created by the spring tension.

years ago DC sold shims to align the ductile adjustables. I believe I have quite a few. PM me if interested and I'll check

Thanks. I originally used those shims to line everything up on the iron heads after grinding down the stamped hold-downs, so I had a good collection to start with. Somewhere there is a baggie with more spares, but I can't find it!

Anyway I took a little too much off one hold-down and had to put a .010 shim back in. So the only shims other than that are the .005 on each side of each rocker Got 'em close enough.

I think you will be OK but like John said I like the spacers instead of the springs

Gus