I have an 85 Z28 with a 305 TPI and stock HEI distributor other than MSD cap and rotor. I was just wondering what modifications/rewiring would need to be done if I wanted to convert the stock coil-in-cap to an external MSD canister coil. Thanks.

Your biggest problem is that there's no HV wire tower for the external coil. I don't think it worth the trouble or expense to fabricate something to make it work. Once you get over that hurdle the ignition wiring is easy.

Personally I would swap to a small cap distributor for a carb model of the same year, just be sure to take the wiring connectors and as much wiring as you can from inside the wiring harness in case you want to reroute it's always better to have more.
Then you can see what's needed to use the MSD coil which I think is nothing more than replacing the OEM coil connector with ring tongue connectors for the MSD Coil's side terminals.

Question, why do you want to use an old school can coil?

They make an "adapter cap" with a terminal for an external coil wire to plug onto the top of it (large cap HEI). I've never used it and don't know who makes it but it's out there somewhere if you want to give it a shot.

search for 'external coil dust cover' on Jegs. Turned up an MSD 8401 and a Taylor 719402.

I was looking at those caps. But didn't know they were plug n play. Will they really work without any wiring modifications?

As far as the HV coil wire to the MSD external coil yes, but you still have to connect the ignition wires to the MS coil terminals, 12v, Ground, and Tach, you know that connector on the side of the cap with the thick Pink wire?