Slow, Driving Rustoration 1972 Dart

Well it's looking like the Dart is getting parked for the winter now. I flushed the coolant system because the coolant had this orange stuff in it. The smell of it didn't ring any bells. I thought it was some kind of coolant sealer so I did a prestone flush on it, and after about 100 miles, it's back, not as bad but it's back. Also I noticed that the trans fluid is getting milky. Yep, trans cooler leakage.

So I really didn't want to park it right now for anything long term but at bare minimum I need to get a new radiator in it, flush the transmission, flush the coolant again, and see what's up. I reckon a transmission replace or rebuild is in its future from what I've read. As far as noticing anything in the transmission going bad I haven't noticed any slipping or noises. I drive like a quintessential little old lady though.

So any suggestions on radiators? Is there any benefit of a Champion 3 row vs a spectra from rock auto? There is about $70 difference between them. $220 shipped for the Champion and $150 for the Spectra.

I've never had overheating issues so far with the stock. The biggest performance upgrade I have planned someday is dual exhaust and a four barrel.

Can I use an external trans cooler instead and keep this radiator?