Why did the po use wood in the battery box?

Probably 2 reasons; to keep the batteries from moving and to prevent galvanic corrosion which is caused from dissimilar metals coming in contact with each other.

 

More than likely, when this truck was new there was ‘hold down brackets’ to prevent the batteries from bouncing around.

 

Also acts as a vibration dampener and electrical insulator, rubber would be better, but wood is easier to find

 

How is the metal in the bottom of said box?

 

Lots of older trucks used wood box floor and battery seperators from the factory. Just like many had plywood cab floors. They actually rotted less then the aluminum and steel ones. Battery acids will eat thru aluminum pretty quickly even if the batts or sitting on rubber mats.

Frieghtliner stiil had plywood battery box floors on some of their models up to 2003. Our bussiness class trucks did.

 

I use a big thick
Mudflap. And a small ratchet strap. Old hold downs alway get corroded and make changing batteries a hassle. A piece of rubber or old memory foam between them doesn’t hurt. Spacing seems to keep them in place, firmly on the bottom. A small piece of wood on top in the middle of both helps also.