There's likely not going to be a simple, bolt-in mounting bracket to adapt that '79 Oldmobile's dash to a modern single-DIN headunit.
The factory unit in that car would have been a dual-shaft deck, right?
You're going to have to do some cutting, friend, unless you go with an aftermarket dual-shaft deck from one of the very few places such a thing can be sourced anymore.
As far as a harness adapter?
I do not know if Scosche or Metra makes one for a car of that vintage...