A guy I know has a 49 Chevy pickup that he did back in the 80's. Battery is behind the passenger seat, and for years he had no issues. About 2 years ago he was wrenching on it in his garage (unrelated to the electrical system) and since the garage door was open and he was under the front of the truck, he didn't realize right away, but smoke started coming out from the cab and was billowing out the garage door. He was able to get the fire out eventually, but the interior was trashed and the paint on the cab was toast.
As near as they could tell when investigating, they think the battery shorted out internally. This is part of the reason I put mine on the firewall. It doesn't look as clean, but it's sure a lot more accessible and there's much less to burn under the hood if something were to happen, and at least it's safer than having something happen inside the passenger compartment when you're driving. I realize this could be a freak thing, but because of this I'm not fond of batteries inside the cab.