Sounds good, although not being there makes explanation difficult especially for me. If you want a 400a alt and 8k soon i'd size the wire correctly now. Redoing it again later will get expensive. For an 8k I'd do 2 main runs of wire front to back (2/0?) myself. Put the best largest secondary battery you can afford in back then ground everything the same as your main run everywhere.
I'm going to lurk instead of commenting awhile, I'm getting confused trying to envision everything with the constant questions. Haha... Since having a stroke awhile back it's my new reality. Not your fault.