I believe the change year from the vertical to the horizontal radiators is somewhere around 82/83. There is a big difference in weight when comparing the vertical radiators to the horizontal radiators. The vertical one in my Argosy is HEAVY and the horizontal one that I removed from the 1986 345 wasn't all that heavy. The 1984 310 that we had came with the horizontal radiator and it never had cooling problems.
I have no idea which radiator is better for cooling.
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)