There is a lot of excessively pessimistic information going on here.
I'm in the midst of building such a rifle myself. I just last night re-read an article by John Barsness in the October '08 issue of "Handloader" (#255).
He used NUF .25/06 brass to start with. He stated that it takes considerable effort/time to trim down the .270 or .280 brass. Secondly, he found that by removing the expander ball from his .270 dies that you could get by using .270dies to reload your ammo. You could simply order a 6.5 expander ball and be good with the .270 dies. I do something similar by loading by .45Colt with a .45acp die set.
I've formed many .257Roberts cases from 8x57 Mauser cases. You may lose a few due to collapsed necks, but, like me you have a large # of cases, so what the heck? I've also made most of my 7mm08 cases from .308's, and .257wby from 7mmRemMag cases. It takes a little figuring out how much lube to use, but too much is better than too little resulting in a stuck case in a die.......
Remember the guys that are talking about a rifle being "shot out" in 2-3,000 shots are talking "match" accuracy as in sub-1/2moa. Not sub 1.5moa for hunting.
If the 6.5/06 is SOO OVER-BORE, what does that say for the .25/06 or .270? Remember, you're talking .007" larger than the .257, and .013" smaller than the .277. Not a lot of difference. The real deal with the .264's is that the twist rates were originally set at ~1/8" so that most 6.5 bullets are longer for weight than the others. Secondly, because of the market for the bullets demand the highest quality (match shooters and "in the know" hunters), there are few sub-standard bullets for the 6.5's. Kinda like there are no "promotional" grade 28ga shotgun ammo available. It's all "competiton" or premium hunting grade ammo.....
If you want a .270 that shoots flatter, penetrates better, and is generally more accurate than "factory standard", then go ahead and build the 6.5.
If "any old" .270 or 7mm08 is good enough; get one of those. I've got a "any old" 7mm08 that will on a good day approach 1.5moa. However, it's a "killing machine", and is my favorite hunting rifle though I've others that are either much more accurate, or powerful.
It's your money and time. Do what you want....