So I have 94 sn95 spindles on my fox w small front discs. Want to upgrade to cobra calipers and 13 inch rotors. Will I have to change my master cylinder for proper function or will the stock one work ok? I'm told that I definitely have to upgrade to add rear discs. Any advise out there?
Then no, you do not have to change your MC because an MC suited for your setup doesn't exist. The stock one is as close as you'll get so you need to live with the pedal feel that you get.
Now, if you want to get pedal feel and brake bias more in line to what is proper, then you should add disk brakes. Plenty of guy do drive around with cobra fronts and rear drums fine and like it