D21 spark plug question

[OSU1]

I have a 1987 D21 4x4 that is missing when idling. I've changed the spark plug wires and the distributor cap (both were in bad need of change). I think the problem may lie with the spark plugs. For my model, there should only be one type of plug, not a plug for the intake and one for the exhaust. Currently, my truck has two different types of plugs in it, neither of which are the correct plug number. I would like to buy four plugs to start out and replace only the side that needs them. I thought I read somewhere that there is a main firing side and a side for either emissions control or when the engine is under load, and since the truck is missing when idling, I thought it should be the main firing side. If anyone knows which side is which, or if you think my problem lies elsewhere, I would greatly appreciate it.