Today's group 34s are nearly the same dimensions as the old group 24s. They are smaller than your group 27. You can look up the dimensions to see it the 34 would be small enough for you. It is a relatively common battery so it shouldn't be difficult to find.

There has been some good information already posted. The cca is the important number to watch. You have an 850 which is a fairly powerful car battery and there are group 34s with that same capacity. But, with capacity comes weight in most cases. So, the most powerful batteries will also be the heaviest until you get into some expensive, specialty batteries.

With batteries like these typically running north of $150 now, we buy them from the late model wrecking yards or better yet, the self serve yards. Most batteries are $30 to $40 there. And you can usually find them that are less than a year old. I have salvage yard batteries in all my cars, truck, tractors, and my Cuda. We get excellent life out of them. Great bang for the buck. The one in the Cuda has been in there for 4 years now.