Jon, I'll give you some unsolicited philosophy
I figure, if and when I ever do a 338, I'll bite the bullet and get a good custom action and chamber it in a 338 LM Imp so I can put some horsepower behind those big 338 pills. In fact, I am thinking of just such a build using a Defiance action, but it's still a dream at this point.
Anyway I've been going through this same mental struggle myself with deciding between a 300 Dakota vs a 300-338 Ultra for my Vanguard/Howa. The bullet I'm using is long and has a very long nose and the COAL in the necked down 338 RUM case is about 3.780 which is way too long for the 2.58 Howa mag box. I was looking at the Rem 700 Wyatt's extended box which is 3.82 and talking with my smith he said it could be done but it would take a lot of work. So I said the heck with it, I'm just going to do it in a 700 action which is a fairly simple install for the extended box. I'll get 10 gr more powder in the case and the brass is about half the price of the Dakota brass.
Back to the 300 Dakota vs the 338 RUM.
Here are a couple drawings. The bodies are very similar dimensions with the RUM being about .230" longer which is about 10% more capacity.
http://stevespages.com/jpg/cd330dakota.jpg
http://stevespages.com/jpg/cd338remingtonultramag.jpg
Have you been following the 338 Sin thread? I think that's the best way to do a 338 in a Howa.