Which would be a better case to adapt to 8mm(.323)? the 300 RUM or the 338RUM? as i understand it the 338 is a shorter case giving long bullets a better chance at fitting in a magum action.

Rock

Well, neck down and you can just run it through a sizer die.
Neck up and you need to fireform. It is said that necking up with expander plugs gives extremely varied results in neck concentricity, thickness.
As far as "longer bullets fitting in a magnum action," it's not a matter of it fitting (you seat it where it goes). Don't confuse yourself into thinking there's less room on the 300 because bulelts will stick into the case body. There's more room there, which is WHY the bullets are sticking in. That's not a problem. But whether the bullet ends up on the cannelure is another question.
With as hard a hitter as this is likely to be, that sounds like a worthy consideration.
Figure out the cannelure-to-tip length of the bullets you want to use, and subtract it from your COL, it's probably going to be more like a 2.8" case or so.
The shorter neck on the 300 is still .3" (the 338 is only .315") which is probably plenty for 8mm, so if you want a little more powder, then go for that one, and with lighter pills you'll have a hair more room. But I'll bet there's PLENTY already in the 338 for your dark plans, I'm sure!