I think the rails are 18ga. but don't take that to the bank. 
18 gauge is more like body panel thickness. The rails should be 14 or 16 gauge in most places.
Swapping the whole rail out isn't as hard as it sounds. Of course, if you don't want to go repop then you'd have to find an orignal. But, because everything is spot welded together, it gives you a map of where the rail should be. Plus, you have the K member, radiator shroud and brace, inner fender etc to help locate the replacement. Obviously, you still have to measure, but the measurements are out there. Keep in mind too that the standards aren't as tight as you think, +/- a 1/4" isn't uncommon. If you have a MIG welder, a flat concrete garage floor, a few measuring instruments and some patience the rail swap isn't that bad, and will usually take LESS welding than a patch.
If you go with caps, anyone buying the car in the future will assume all the rails are swiss cheese underneath them. And, they might not be wrong. Not saying that a good repair can't be done using caps, but they can also be used to hide a bad, or NO repair at all. If the car is rotisserie blasted, the rust removed, and the caps applied and sealed up, the rust should stay away. But that's not how caps are usually done, in many cases they're slapped on over the rusty rails and welded on with minimal treatment to the original rail. When used like that, they're a temporary fix. And most buyer's will assume that's what was done. Plus, they're really obvious. Maybe not driving down the road, but anyone that inspects the car beyond a casual glance will notice them.
If you really don't want to mess with the whole rail, or only one section is bad, then I'd cut out the section until you hit good metal. Then I'd clean and prep as much of the surrounding area, including the inside of the rail, for as far as I could get. Then, use the method Final Challenge mentioned. I use manilla folders usually, they're cheap, closer to the right thickness, and flexible. Trace out the patch on the card stock and transfer it to metal, leaving the patch a little too big. Then just trim it until it fits and weld it in place. Done properly, it should be as strong as the original rail. If you're concerned, you can always reinforce it with a secondary patch over the top, like a gusset plate. But, keep in mind this will change how the frame flexes too. And, keep in mind that once you start cutting, you may end up taking out more metal to get to good rail than you want. If you're patching more than 6-8" of rail, or having to replace 3 sides of the channel for any length, its going to get more difficult.
