You don't want to piggyback over a custom ECM tune. That's the old school way of doing things back when tuners didn't have the know how or access to the correct tables to create a good tune.
I'd like to see GM's book hours for that trans drop to replace the belt. I've heard a few say it's a bit easier to do it GM's way rather than how Ford does the new 3.0L.. so, pick your poison! :D