While its difficult to tell from one fuzzy pic, I'm not sure that you need a whole rail. My car's drivers front rail is a bit "soft" under the battery area as well, but sound elsewhere, so I'm just going to media blast it and weld in a patch. How correct you want to be with that is up to you, you can either cut a section out of the frame and weld in the patch flush, or just weld the patch over the top (kinda like a cap) if you're not worried about it appearing stock. Some surface rust is no big deal, all these old cars have that, just be concerned about holes or major pitting. I used to do a lot of patch work when I was in Austin Healey restoration, lots of thin metal and rocker rust on those, and the patches can be very well executed if you take your time making the pattern. Also, replacing any whole rail is pretty time consuming, as you will have to do a lot of measuring to make sure that your suspension ends up in the right place. Driving a crooked car is no fun, and even if you set it all up right sometimes things move with the heat of welding. Not too likely with all the spot welds on these cars, but I'm sure its happened.
