Howard,
I think it all depends on how much of the reinforcement you need to change. If you just need say the front two thirds or maybe a little more you could in theory splice in a section but if you need to replace the entire reinforcement you will need to remove the fender.
In the first photo you will see where they usually rot out across the top and in the second photo you will see how deep into the structure the rear of it is. You would have to drill all the spot welds from inside the engine compartment, cut the ends and slip the new piece in. The caveat is wherever you splice it in you will not be able to weld across the top of the reinforcement as it will be too tight to get in there with a welder. It "might" be possible to cut a 3" long window in the bottom surface and weld the top from inside the rail then weld in the window in at the bottom.
It is all going to be a pretty miserable job so patience is a must.