I would hazard a guess that 1987 would use the same fuel tanks as 1990. If so, fuel tank capacities are as follows:
Standard - 16.3
Optional - 20.0 (Only available on 114 inch wheel base)
Supercab - 16.3
For reference, wheelbases are as follows:
107.9 - Regular Cab, short box
113.9 - Regular Cab, long box
125.0 - Super Cab
Wheelbases are from Wikipedia. Fuel tank capacities from 1990 Owner's Manual.