how bad is this rust?

what does the inner fender look like?rusty around the battery tray area?
if the inner fender is soild and there are no other major rust issues I'd
say go for it. pull engine and tranny and remove everything from firewall forward. get a couple of spot weld cutters and go at it. I do know that you need to make sure you get the replacement rail right back where
the piece of crap was, take lots of measurements at cross member and
radiator support and between rails and of course as 72bluNblu said diagonally. if you leave in the inner fender and do above mentioned you should be alright. Do you have a garge to work in?