The way that I look at it is this:
If your going to go through the headache of pulling the head, o-ring the damn thing.
Its an extra $300 to have it done at most places. You'll need to fluxed and decked, but that usually only runs an extra $100 or so.
I just went with a .020 12v gasket from Mumau Diesel. GREAT people to work with. Everyone who has done this has said that they lose some bottom end.. I haven't noticed that at all. I have noticed that the truck starts a little harder in this cold weather. When I say starts harder, what I mean is that I actually hear half a crank. Before, the second I touched the key the truck fired off. Now, I hear about a 1/4 second of "ruu" before it fires off. I'm also attributing a little of this to the fact that these are the original batteries from 2001.. And I think they are about due.
Basically, no matter what gasket you decide to go with, I would at least get the head o-ringed. If you don't have studs, get them too. If you're going through the trouble of doing this, do it right the first time. You should be fine with a factory 24v cummins gasket, but if you want to get a modified 12v I would talk to Mumau. No matter what gasket you get, I would presonally recommend an original CUMMINS part.. I wouldn't go with any of the non OEM parts offered.