I don't do alot of Magnum stuff, mostly because of all these issues, you run into with the 8MM deal, & valvetrain issues, but...I've had to fight with enough of it, to have some knowledge.....

Like Patrick suggested if you are using 8MM Magnum valves that use a round lock groove, those valves are setup with very short tip length, to use a magnum style lock which is essentially similar to a Nascar style "top lock" Basically the lock groove is all way at top of lock, ie. there is no such thing as a +.050" lock for a Magnum style valve. Same deal on the retainer, The whole idea of using that top lock design was to get IH as tall as possible, so you're going to run out of rocker to retainer clearance if you try to run some type of +.050" retainer that stick well up above tip of valve.

So IMO, using Magnum valves, you can throw out the idea of adding +.100" of IH from different locks/retainers.

I've never cut apart an RHS head so I can't make any comments on how far you could machine spring pockets down.

This lack of IH issue, is not really "new" to anything Magnum related, it's one of reasons I prefer to use Magnum Edelbrocks...... It uses a standard SB Chevy 11/32" valve, with traditional .250" tip length & a square lock groove. Also they come through setup for a 1.80"+ IH right out of box.

I've had some guys wanting to have me setup Magnum R/T's for .700" solid rollers..... Good luck.

Usually we end up going to LS1 valves, but that only helps so much..... it's just an inherent design flaw of those heads.