I think Deuce's underfloor setup is a very safe way to go.
All you need is a sufficiently strong battery box that'll meet the tech requirements.

Keep in mind, a poorly retained battery in the trunk is nothing more than a big rock flying around if it comes loose in a rollover.

The battery in my 32 roadster - and the 31 roadster - sits in a 1/2" x 1" rectangular tubing box that bolts to the frame with four 3/8" bolts.
It's not all that difficult to access.

Interesting part about running the battery outside of the engine compartment away from the heat, you don't lose much battery water.
I only added water twice in an over three year period when I ran a standard battery.
Now I run an Optima and no probs.

The battery is cushioned top and bottom by 1/2" thick neoprene pads and it has a 1/4" x 1" aluminum strap hold-down up top - also neoprene padded.
Trouble-free for the last 14 years and 48,000 miles.