Magnum replacement heads

The Mopar magnum valve stem size isn’t really “correct” for 5/16” or 8mm.

For “stock replacement” valves, the stems are listed as .3115.
8mm is .3150.
So if the guide were exactly 8mm the clearance would be .0035”. Too loose.
5/16” is .3125”...... which would yield .0010” clearance. Too tight.

By comparison, a stock replacement LS valve, which are also 8mm, has a stem diameter of .3136.
The guide bore is listed as .3154...... clearance is .0018”....... perfect.

So, it seems the aftermarket magnum heads are using “standard” 8mm sizing...... which isn’t what the factory used.

Someone wasn’t paying attention when they specified the guide sizing on the new heads.
Ideally the guides would be .3125-.3130, so that the final sizing could be set by the shop finishing the heads with a simple honing.

Fortunately(?) installing liners will allow you to set the guide clearance to what you want.

As for the flow, I can’t help you with that without having one here to test.

As a side note, I had a set of lightly used EQ’s in the shop here a couple years ago(I rarely ever see any magnum stuff).
The guide clearance was pretty loose, and the guide bores were very rough and poorly finished. They got a set of 16 liners installed.