Some guys use products like CLR to remove scale inside the radiator.
But if yours leaks, that is not good and it might leak even more after doing that (you will unclog all the pin holes leaks).
You are also losing cooling ability by not having the system quite at full pressure (each PSI increases the effective boiling point by 3*F).
For the birdie radiator, like I said the info was just to be taken for what it's worth. A quick eyeballling of one is worth it, but they had a small front end just like stangs.
I have come across one guy who said he used a HD fox 3 core radiator in an SN. But that radiator now costs about what an alum universal radiator does and would likely require just as much fabrication to fit.
I am trying to think of budget ideas and not doing so hot. You can call a rad shop or 10 and see what they say. I kinda doubt they can do much with our stock radiators since they have plastic tanks. Maybe they have or can whip something up that would fit and not cost too much. It might be worth taking 10 mins to call.
A lot of online rad places have free shipping, lifetime warranty and cheap prices.
Good luck.