1. Inverters. One word? DON'T! 4kW-5kW off a 12vdc vehicle battery? - I make that over 400A. Vehicle batteries are not designed for this type of application and will fail pronto-they like a short burst and immediate full recharge. You need separate 'deep-draw' batteries (ie golf-cart type). These are cheap, but have a limited life. You should only discharge them 50% for longer life- even then you'll be lucky to get over 300 charges before new ones required....
2. Run a gennie on the V8. Sounds good on paper, but.......Gasoline engine= no governor= variable speed at the alternator shaft= fluctuating hz. and volts. That won't bother power tools/wellpump, but don't plug your computer into it! Also, you'd be heaving the alternator round all the time when not using it- no promlem for the V8, but an alternator running at 2 or 3 times its design speed when you gun the V8 might have. Less mpg too, the alternator needs power even on idle for its cooling fan. Not sure how you get speed contol -hand throttle? -on a modern engine with electronic injection, and of course there's the engine temperature to consider= electric fan almost mandatory. But it would work, the alternator will be a lot cheaper and lighter than inverters and batteries. With all the above limitations in mind, for 5kw (6.7hp) youll need duplex (2)x 1/2" vee belts = custom crank pulley, duplex alternator pulley. Bracket needs to be strong, aligned properly & adjustable for belt tension, with room round the alternator for air/access. Make sure belts have good arc of contact. As to the 120-240V AC wiring /switchgear/ breakers/ grounding out of the gennie, I can't comment as I'm English, but I'm sure the members will post some good advice- so watch this space!
Wood work but can't!