Don't forget that the chamber neck is longer than the case neck. So you might need to set it back farther than .25". Several people sell the 30 gibbs reamer I haven't seen one for rent except maybe at Shawnie. If you can find a gibbs in another caliber with a removable pilot you could use it and a 30 neck/throater. You might also check with http://www.hunting-rifles.com/. Their 300 Howell moves the shoulder forward so it might help.

If the diameter of your neck is to large going with a 30Gibbs will not change that fact unless you have room to set the barrel back far enough to completly recut the neck. If you set the barrel back that far wouldn't an 06 reamer do the same thing? The length of the Gibbs case is the same as the 06.

I'd need to set the barrel back by the length of the neck, inorder to get into fresh metal. Gibbs uses a .25 long neck, the -06 is somthing like 3/8+. To go that far (returning to a 30-06) would require removing the barrel shank and re-threading (there is enough metal to do that if required, but it's not the desired methiod).