I agree. IMO, it would be best if not just simply perfect to get a new 3 core and factory shorud and viscous fan for this set up. Use distilled water and peak anti frezzeee.
A 2 core from a smaller engine will cool it only so much. My /6 rad cooled my 318 for the most aprt but it did have trouble after a while runing around town. It was like the rad. was just barley under the min. requirement for a mild 318.
It was a new rad and electric fan w/shroud set up. Probably be great for a hopped up /6.