There's nothing wrong with a 3EE battery, I just prefer a smaller battery (I use a size 25 in my '64). Modern technology has given us stronger, more reliable batteries in a smaller package, and I generally go with modern technology. Studebaker did, in '64, changing to a size 24. A size 24 is a car battery, a 3EE is a commercial battery.
"Diehard" is just a marketing label. I don't know who makes their batteries for them.
Actually, mfg has (I think) the ultimate solution: a gutted 3EE in the engine compartment and a size 24 in the trunk. Several of our SDC Forum experts have advocated relocating the battery to the trunk.
To those who keep their '63 Avantis original, I say more power to them. I do like seeing all-original Studes at shows, with bias-ply tires, etc. I'm just not into 3EE batteries and bias-ply tires.
--Dwight