If we didn't keep everything from the 89 IROC (much like the cook who eats his own pie, or more like a junky who uses his own stash) - there was around $5000 worth of goodies on that IROC.
After combining the 2 - you'll have about 1 or 2 months of gas money too, or your 1st semester college tuition.
The only PITA you might have to deal with is putting the wiring from the IROC into the 86. Let's assume (hopefully for you) - your IROC has the L98 350 - the 86 Z will have a 305 TPI at best, or the LG4 305. You might be able to get the LB9 wiring harness to fire the L98.
The other issue, with the wiring, the 89 should have VATS, and an electric speedo, while the 86 is still cable driven, and has no theft system built into the wiring. Keep this in mind if you swap transmissions around.
The 86 Z28 and the 89 IROC should be identical looking, externally, with the exception the rear spoiler is thicker on the late 80's to include the center brake light. On the 86 the brake light is on top of your rear window and looks diq.