Oh, BTW, your '86 will have the mounting spot for the ECM, but you'll need to find the nylon(?) base and retaining piece out of an '87-up truck to make it fit as stock.
Just an FYI, it pays to mount the ECM firmly, although you'll have to mount the relays somewhere else, which is no big deal. The stock mount is easy to come by, and does a good job, I really recommend it vs trying to cobble something else together.
If you want it to be a clean install, you can take the grommet for the heater wires out, (de-pin the heater wires from the plug on the heater box) make the cutout in the heater wire grommet bigger to fit the additional wires, de/re-pin the ECM connectors (one wire at a time so you don't get interrupted and forget) route the wires through the heater grommet, then install it. It's easy, just takes a few minutes and a paperclip. I did this not too long ago with the '88 Camaro wiring, it fit great, and seals good too.
I wasn't about to drill a new hole, and besides, it was easier than trying to come up with a way to route the wires so they wouldn't get cut.