Measuring for a radiator

71 dart. not sure what motor it originally can with. Was not a 340 for sure. When measuring for a new radiator, do you measure between the holes in the core support or ??? Also, need an idea on whether to buy a 3 or 4 core. alum or regular eom style. Engine will be a 5.9 magnum, with the factory f.i. and stock camshaft. I plan on adding an a/c kit later. TIA all.