I'd sell the 280z and buy a 240z.
Install all your parts in the Z, sell off the rest and get your garage back. I think the Z cost less to insure, register, maintain, etc.... than the A6. It just costs you time, but you can turn that into good memories with your kids.
You even said, financially its one or the other, and the Z will cost less( unless you do a full resto on it). And if you've had it that long, you will get the itch later on and probably regret selling it.