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Check out Speedway motors. Someone posted here about 6 months ago that they used an aluminum radiator from Speedway. I think it was less than $200. They had several different sizes. You might have to look in the race car side of their catalog instead of the hot rod side.
I just put a PRC aluminum single pass in my '57. 27.5" wide x 19" tall. Simple piece of tubing for a bottom mount and a small bracket for the top mount. Mounted a Be Cool tranny cooler to the bottom side of the same mount. They run around $175. Be sure that any aluminum radiator you buy has a billet neck and not a stamped piece. They work well to cool our Yates headed dirt late model motors, I have total faith in it cooling my mild SBF.
Also look at using a 19" high pitch 4 blade fan if you want the best cooling. We also run 25# radiator caps. Around 700 HP and it runs 190 all night long, all summer without a single hot night.
Priced a new one from NAPA and they want $350.00 for a new mid 70's Dodge pickup rad. I think I'll check a few of the wreckers first.
I bought my 72-79 Dodge pickup 3-row radiator from "Completeradiators" on Ebay for $159 plus $15 shipping. Look at item 330158762373 on Ebay right now for $143.08...even cheaper and new in the box but I don't know for sure if it's a 3-row. There are others...