Been searching while me fingers are thawing out and I came up with this.
BCI group 1 6v battery is 9 1/8" x 7 1/8" x 9 3/8", CCA 650, CA 850
BCI group 86 12v battery is 9 1/16" x 6 13/16" x 8", CCA 540, CA 675.
Terminals both on one side and they're on top as in Gus's photo.
Don't know what group that bat is that's in your photo Gus but
maybe you can post it for a comparison if it's different.
So that battery is gonna fit with a few fractions to spare,
only not have quite as much stamina as the 6v Group 1. But
since the 6v starter is gonna get an extra kick form the 12v
battery it shouldn't present a problem, unless you're in dire
need of a tune-up.
Denny Graham
Sandwich, IL
