The short answer is NO...your existing
12 volt battery is most likely a Group-27 model, which is about 12 inches long, by about 6-1/2 inches wide...
The 6 volt (golf car) deep cycle batteries have a 'footprint' of 10 inches x 7 inches wide - EACH. so you will need a space of about 20 x 7 for end-to-end, or a space of 14 x10 for side-by-side mounting.
ALSO, the 6 volt's are about an 1-1/2 taller than the 12 volt's, so you'll need more clearance as well...
The 6 volt's are great for use in RV's, if you have the room...you'll get about 225 total Amp Hours of capacity, where as the G-27, 12 volt has about 100 Amp Hrs of capacity...
Hope that helps...Ray