using 2 lawn mower batteries

be careful placing batteries inside the car. I'd be concerned about proper venting of the batteries.. they produce explosive gas... worse than the driver... a spark of any kind inside the car could kill yah...

 

Miata battery cranks my banger right up.

 

I used a 350 CA/300 CCA lawn tractor battery in my HA/GR. Spins the flathead six over great. Cold cranking was no factor for me, the dragstrips are all closed when it's below freezing

Keep in mind that your V8 should never need more than about 175-200 amps to crank over. If it takes more than that, you have issues.

Reserve capacity is important too, how long do you plan on having to crank before the engine fires?

$29.95 at walmart

 

Here's my optima mounted under the
passenger side of the seat in my truck.
maybe you have room for something similar.

 

Try an Odyssey gel cell...small, compact and plenty powerful. And not quite $200 either http://www.amazon.com/Odyssey-PC680MJT-Automotive-LTV-Battery/dp/B000XEF3DM

 

The long skinny battery in 12 Volts is a Group Size 3EE. The Old Roadster had one in it sitting sideways on the rear crossmember just behind the gas tank since it was built in
1969. Never a problem, still available everywhere, modern use in forklifts.