In all likelihood it probably will, but only way to know for sure is to compare the part numbers for a radiator for both trucks, or try any auto parts website and see what radiator they recommend for each vehicle, if they're the same then you could assume that your 1990 radiator will work in the 87. This also assumes you have the same engine in both vehicles.
Search Isuzu Pickup Questions
Isuzu Pickup Experts
Find great deals from top-rated dealersTMSearch