Thanks Guys, kind of what I was figuring when I thought more about it since the connection, under the pretty stripped donor van hood, was to the fuse box bus bar and not another battery (or where I thought the battery would be).
So, if I decide to use both these factory frame mount boxes on my 2003, it looks as if I will have to drill new holes in the frame and I will have to use Group 65 batteries.
Alternatively, I would get a new battery tray fabricated (ala Nuther Battery Box) and then could use any batteries that would fit and be able to utilize the existing holes which are about 27 inches apart. (One set is 15" for the a single box, plus another set of holes about 12 to 13 inches further on) Not to excited about adding to the Swiss cheese feel of the frame.
Open to suggestions / advice on which way to go
largest load is 900w Microwave used very sparingly, and Norcold MRTF60 Fridge / Freezer. Some LED lights, laptop, small fan when hot. Two Grp 65 AGM's 20hr@75A could work but it seems it would be much nicer having two 20Hr@100Ah+ batteries (or single 4D).
carringb - Yes these will be for a two battery house bank. I want simple, so it maybe a Ford Aux battery relay, or simply a heavy duty manual battery switch which I can turn off and on to separate the house from starter. Let me know if anyone has a good condition Ford Aux Relay sitting around. I will be installing fuse / breakers for protection.
Already have an inverter and will just add a battery charger into the mix.