there are a couple of different routes you can go.
The first on is a bit more involved, but is the most 'correct' way. It involves swapping the entire core support with the earlier one. in this case, everything just changes over. the core support is welded in, i believe.
The most common way I have seen though, is to just cut the Core support on the 78+ van back to accept the early grill. then you may have to cut and drill some holes to get the lights in the right places.