Are you sure it was just a 'Y' pipe? Why I ask is the 3.9L that year may have had a light off catalytic converter (or two) just under the exhaust manifolds.looking for walkers part number for a y-pipe for a 1988 dodge d150 with 3.9. Doesn't come up in new books, does anybody have a older cross reference.
Interesting since at least the early 1988 360's were non computer controlled Q-jet carb motors with vacuum advance ignitions.Looking at the factory parts book, it seems to say that only the 360 has the front converter so it is possible the 3.9 uses simply a Y pipe. Somewhere I have an old exhaust catalog, I'll have to see if I can find it tonight.