Intake/Exhaust valves

I haven't really played with these yet - but the retainer is the least of your worries when you go with longer valves and yes, a beehive would help with that interference. but that's not the big problem. The geometry with a shaft mounted rocker system will be off with longer valves because the valves tip slightly towatrds the shaft centerline. So the longer the valves, the less distance between the shaft and the valve tip. that's the bigger issue. I don't know the angles on the slants so I can't say how bad that would be. But - the fix for that is to move the shaft centerline. Sometimes that can be addressed with rocker support blocks or just shims if it's not a high rpm engine.
You also get to the point where the port can't flow what the valve can anyway - which makes it all a moot point. The ports suck.