Lucille:

Yours looks exactly like the one I bought from Chevs of the 40s. As JiMerit said there should be a long bolt through the two 3/8” holes at the left in your photo to clamp the ends of the battery. I also placed a matching bolt in the holes on the frame side of the unit.

I’m old enough to have dim memories of under floor batteries and do not recall any straps designed to hold them down.

Depending on what size battery you end up sing you will have to make some wooden filler pieces to take up the spaces around and maybe under the battery. Also be careful the posts and cable clamps do not touch the cab floor.

1941 Chevy Master 3/4-Ton
In the Gallery
More images in the Bolt Bucket

Born to fish, the truck is only a hobby.