The 300 winchester magnum is 156 thou' longer from base to shoulder than the 264 win mag. Otherwise the cases are very similar. Would forming 264 brass from 300 brass be a reasonable proposition?
The reason I ask is that Lapua makes 300 win mag brass, but not in 264, and I really like Lapua brass.

I see no reason why it should not work. One will probably have to ream case necks as well as anneal them.

Does Lapua make 7m/m Remington Magnum brass? If so, this would be strictly a resize job, -much easier.

Yes, but even easier would be forming it from 7mm Rem mag., as less trimming would be required.

The easiest way is just to buy .264 Winchester brass.

Why bother with case forming, fireforming, reaming and annealing when you don't have to?

Someone else has posed a similar question about nickeled .300 brass. While I would be a bit hesitant to try this conversion with nickeled brass (due to its generally harder nature and the addtional stress of setting back the shoulder), if you want to use regular un-plated .300 brass, it should present no special problems, just extra work.

As others suggest, starting with 7mm Rem Mag, or even .308 Norma, would make life simpler.

That said, of hundreds of game animals and thousands of varmints that I've done in, none seem to be any less dead because they weren't taken with Lapua brass.

I would use 7mm Remington Magnum brass if Lapua made it, but they don't. They do make 300 Winchester Magnum. Thanks for the advice. As for the other comments:
I predominantly shoot F-class. I wanted something to rechamber an old 6.5mm barrel to. It won't be my number one gun, but 264 Win Mag promises to be some fun. I don't use Winchester brass for target work any more because it just won't shoot as well as Lapua brass, no matter how much work you put into case preparation. And once your WW cases are any good, the neck will split or the case head will separate long before the Lapua brass ever will.
If it costs twice as much but lasts twice as long, it has to be at least as good.
If it shoots better, it is better.
Winchester brass is perfect for hunting: load it up and then leave it on the ground once it's shot.