I thought my T-1275s weighed 82lbs each, but didn't look it up just now.
My previously enjoyed T-1275s (made in 2011) are still in good shape since I restored them two years ago after they were in a golf car.
I expect the T-1275 would be a good candidate to replace the 31, but it will have its own personality of course.
They do act differently from my 6s in how they react to loads (Niner mentioned this too ISTR) The T-1275 takes loads with a voltage drop like you would expect (not as big a drop as the 6s), but when the load is removed the voltage bounce back is sort of relaxed coming back up, taking longer than it does with 6s. This might worry you at first, but it is normal.
They don't use much water. I kind of like my T-1275s and have come to trust them, which is the main thing- you want a battery you can trust.
