You may want to Check Champion rads as well , that one seems to be a bit narrow as the factory core width was 26"or 22" it seems to be a universal not a direct fit or maybe I am reading it wrong .
there could be a number of other issues , timing should be 12-16* at idle , but not over advancing past 36* total , vacuum advance is best left disconnected but must be connected to ported vacuum, low & idle & increasing with RPM opposite of manifold vacuum , is the fan on backwards , I have seem this often , is the thermostat working OK , boil it to check it . Add a shroud & have a good 18" clutch fan . often the clutch gets weak & will not pull much air . has the rad been checked for flow , just cleaning it will help . is there a spring inside the lower hose to pervent it collapsing with suction . lots to check .
Adding a external trans cooler is easy , just connect the 2 lines to a seperate cooler & mount the cooler in front of the rad