What is the proper way to permanently spread the rear dropouts just a few mm. on a chromoly frame?

Oh, by the way this for a freshly restored frame, so looking for a method that doesn't use a torch, dynamite, etc.. (in other words, trying to avoid finish/ dropout damage).

I know bike shops & road bike guys do this all the time, just wondering HOW. Thanks!

A couple of mm should be no problem. Grab an axle and four axle nuts. put the axle with 2 nuts in dropouts with nuts on inside. thread nuts on outside of dropouts and tighten down one side only. leave the other outside nut a full rotation loose and tighten the inside nut to spread the dropout. lossen outside nut again and repeat till you are roughly 5 mm over where you want it (as the frame will spring back slightly) and voila spaced accordingly. do this slowly and carefully measuring your frame periodically to make sure it is spreading evenly.

Cool, that's how I figured I should do it, I just wasn't sure. Thanks for the advice!

Not too sure about that, as the dropouts are attached to the seat stay and chain stays (on a looptail, they are one) you cannot permanently adjust them without tweaking the frame.
You can however, take a large crescent wrench and adjust it so it just fits over the dropout ends (upper and lower individually in relation to the axle slot) and bend them outwards, but this may indeed be asking for trouble unless your adjustments are very minute.
There's a couple ways around this, the most solid method being to adjust the spacing on the rear hub by removing washers/spacers and installing thinner ones from the ole parts bin or hardware store to get the 'perfect spacing'. This may be time consuming.
If it is just barely off, then either have a second hand spread the dropouts as you slide the wheel/hub combo in place or use a 'quick grip' clamp with the arms reversed as a spreader to widen the gap slightly, then install the wheel.
Or if your patient enough, use all four limbs to spread the droupots while carefully installing the wheel in the rear dropouts without marring the dropouts if you're a total perfectionist.
Better yet, just space the hub correctly to the frame as mentioned above using a variety of washers from your parts bin or even filing down the spacers carefully. Rather, modify the hub and never the frame.