I believe there is a slight difference on the fenders where the valance bolts up between them but it is not enough to keep you from using either style valance or even noticing from one side to the other. There is no difference around the headlight trim and nothing that would impact swapping from 4 lights to 2. That is more a function of the core support and headlight buckets. I think you might have to drill a hole or two to go from 4 lights to two, but the structures of the core support are the same.
The bottom clips of the later 2 light style grill do fit into slots in the later style valance so you might have to get creative to hold the grill solid if you want to use a stock 2 light grill along with the early valance. Should be easy enough to fabricate the needed parts, just depends on what parts you want to mix and match.
There are very few major body changes all the way from 72 to 80, and most of them are the tail gate and tail lights.
_________________
95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)
MEPR: Man, my 4x4 makes all other LUVs look good