Are the igniters interchangeable?
 

If I go to the junkyard tomorrow, can I get either the leading or the trailing ignitor to replace the suspected bad leading igniter?

Or do I have to get the "right" one?

(And yes, Pull-A-Part is open on the Fourth, before anybody asks.)

Yep.
In fact,a quick roadside fix if your leading ignitor fails,is to swap the trailing ignitor into its place.The engine will run fine with the trailing system off,but not the leading......

Good point. Just so you know, the 81-85's had a J-109 ignitor, while the 80's had a J-108. An 81 will not fit an 80, and vice versa. 79 of course still had a points system, so no ignitors.

I am thinking about just using a TII leading coilpack, an FB coil, my MSD box, and ditching the ignitor.

But now I have some sort of partial throttle problem (vac leak?) so I might not even get the engine hot enough to test if another ignitor fixes my missing problem.