Or check out Sheldon's page about rear axle spacing and how to cold set a steel frame. If you know this frame will be dedicated to single speed or fixie riding from now to forever that's how I'd do it. If you want to keep your options open then go for the 5 mm spacers under the inner locknuts. As mentioned you may need a new longer axle. Don't go with the original unless the axle comes out at least flush with the outside of the stock axle nuts. Any less and you're risking stripping the threads over the long term. Mind you if it's about 3/4 of the way into the axle nuts it'll be OK for less strenuous riding for long enough to get to your LBS and get a longer axle. But don't torque it fully and don't do any steep hills or sprints.