Is your 351 a fresh overhaul? Is it bored out,especailly over .030? If both is true, I would lean towards finding a stock (to the truck) 4 row radiator. The reason I say stock to the truck is a stock style unit will get all the airflow the grille and hood was designed to guide threw the radiator with no modifications. You can put a lot of different radiators in the truck and they may work, but your goal is efficent,focused airflow through the radiator. If you do go the route of a non OE style, make sure you build it to seal the gaps all around the radiator mounting so air flow can not by-pass the radiator. Driving down the road there will lot lots of air flowing, but in stop and go traffic, a radiator that is justed mounted in the truck may let air leak past, thus getting you less airflow pulling thru the raidator by the fan. Speaking of fans, hopefully you have a 12v system if for no other reason you could use an electric fan and schroud combo. I do not think I am an expert on this. I had a simuliar situation like this on my '56 just last summer. I have a .040 bored engine and it likes to run hot! They say the more bore, the less surface area the engine has for cooling between the cylinder bore and water jacket. I tried everything, even other radiators, but the key seemed to be to use one built for the truck. And switching to an electric fan also really helped. Anyway that was just my experience.

 

I would think it's a stock 351C. It is coming out of 1971 LTD wagon. So probably very stock. And yes I will seal around the radiator. Thank you so much for the post.