We have been asked this question time and time again.  Here is the response, including our typically explicit pictures.

Many of today's BCI Group No. 1 6-Volt battery terminal heights come relatively close to the inside face of the battery box cover and often do not fit with original style rubber bumpers in place.  With the addition of replacement battery cable ends, the height is increased even closer to the lid, often shorting the battery to its metal surface.

The battery that we use is a BCI Group No. 19L from NAPA Auto Parts.  The part number is BAT5019L, and the design specifications are 8 1/4" long x 6 3/4" wide x 7 1/2" high. This automotive battery is used primarily for 1958 to 1969 BMW, MG Midget, Opel, Volkswagen and Volvo. We use NAPA brand because it is the closet supplier and it works for use.  We have heard there are others on the market, specifically Wal-mart and Tractor Supply, but have not confirmed their sizes or fit.

The battery boxes shown below are not aftermarket "extended height" battery boxes.  (The two upper pictures are the aftermarket replacement we sell, and the one installed on the 1947 Cub tractor is an original battery box.)

We use the NAPA battery on our tractors and do not have to remove the rubber bumpers from the battery box top covers. We place the battery on a 1/4" thick piece of rubber for added protection and better top fit.