I'd buy the alt 1st, grab wire next. Run as is for a bit then park the car. Cut to length and deal with terminals correctly. It's going to 5k-8K? DIY it and buy welding wire I say. Dual 4/0 welding may be a bit inflexible but that copper once crimped correctly won't fail. I like to run things underneath and stealth myself. A hole in the trunk or floorboard is preferable to stuffing huge wire under carpet and running it different should relieve the bends a little. Install 2/0-4/0 welding wire once and you'll see... lmao.
I'd personally run dual welding wire 4/0 runs for the main run, dual 2/0 welding wire on the alt /w/ dual lug and match or exceed the mains at the amp and grounds front and rear.
400a alt on 5-8K correct? Dual U-12s on big power?
Poor Altima