Not 100% true. The Ruger M77 257 Roberts is built on the long action. The 257 Roberts swings both ways LOL.
It's hard to give you an answer. You don't mention bbl contour or how long your barrel shank is.
If it's to small in diameter to cut the original threaded length off and rechamber deep enough to clean up the old chamber completely, it's a no go.
IMHO, take a 25-06 and a 257 Rob case and measure their approximate lengths from the bolt face, out along the barrel. Check the thread tenon length as well and mark where it will need to be cut off to clean up the chamber as well.
This should give you a pretty good idea of whether you can get away with the conversion.
IMHO, it should be doable if you have a standard taper barrel.
I've done this conversion on take off Remington barrels without any issues.
As for action length, the long action is preferable and you obviously have it.
One thing about your existing barrel, it likely has a 1=10 or faster twist rate to stabilize the heavier 257 diameter bullets.
With decent hand loads, the 257 Rob will come very close to factory 26-06 velocities with some of the lighter bullet weights.
I really like the 257 Roberts. It's a very efficient cartridge.