I don't really see why the barrel would need to be over 24", the 30-06
case is not that big & the 308 bore gives it a good expansion ratio. You
will have some gain from 22" to 24", but the gain from 24"-26" would be
minimal with most loads, but of course you would gain some.
The same basic case with a smaller bore would gain a little more like a
270, or 280 & for sure the 25-06, in other words the smaller the bore with
the 30-06 case, the more sense it makes. By the same token the mags. can be helped alot with longer barrels.
If you insist on a 26" tube for a 30-06, you can get it in a Ruger #1, or a
TC Encore, even 28" in the Encore. If for a bolt gun, you probably will have to go custom.