I also did this upgrade to my 1981.
I put the full super cooling Radiator in mine.
The Radiator support will bolt a narrow or wide radiator in place.
However, it is adviseable to swap radiator supports on truck with the narrow radiator, because the narrower radiator support has a smaller opening up front than the "wider" support does. The extra opening for the larger radiator lets more air to the fins, therefore cools even better.
Then you can upgrade the fan guard to a fan shroud, and replace the flexfan with a fan clutch as well.
Very happy with my conversion, and it cools really well. And it's all stock parts.
Picks of radiator, shroud, and fan clutch are in my gallery.