Rule of thumb for finned area of radiator is determined by cubic inch of the motor. Example........if a motor is 400 cu.in. then radiator should be 20"x20".
Now this is not set in stone but used as a reference. HP is a biggie as well. More HP= more heat. Tube width inside radiator can also be a big help. Aluminum rads usually start at 2 rows of 1" tube. I have seen custom cores made that are one row of 1.50" . So there are choices if you look into things.