This is quick & rough, and I don't have previous addition bronto stats anywhere, but by comparing other dinos in the 3.5 SRD against one another I come up with something like the following. Bronto weighed about 30 tons (as compared to something in the area of 100 tons for seismosaurus if I recall correctly). BTW, I don't know whether we care or not, but bronto's name was officially changed to Apatosaurus some years back.
Brontosaurus
Gargantuan Animal
Hit Dice: 24D8+192 (309 hp)
Initiative: -1
Speed: 20 ft. (4 squares)
Armor Class: 13 (-1 dex, +8 natural, -4 size), touch 5, flat-footed 13
Base Attack/Grapple: +18/+41
Attack: Tail slap +29 melee (2d8+16)
Full Attack: Tail slap +29 melee (2d8+16)
Space/Reach: 20 ft./15 ft.
Special Attacks: Trample 2d6+16
Special Qualities: Low-light vision, scent
Saves: Fort +22, Ref +13, Will +9
Abilities: Str 33, Dex 9, Con 27, Int 1, Wis 13, Cha 12
Skills: Listen +?, Spot +?
Feats: Alertness, Diehard, Endurance, Improved Bull Rush, Power Attack, Run, Toughness (3)
Brontosaurus
Gargantuan Animal
Hit Dice: 24D8+192 (309 hp)
Initiative: -1
Speed: 20 ft. (4 squares)
Armor Class: 13 (-1 dex, +8 natural, -4 size), touch 5, flat-footed 13
Base Attack/Grapple: +18/+41
Attack: Tail slap +29 melee (2d8+16)
Full Attack: Tail slap +29 melee (2d8+16)
Space/Reach: 20 ft./15 ft.
Special Attacks: Trample 2d6+16
Special Qualities: Low-light vision, scent
Saves: Fort +22, Ref +13, Will +9
Abilities: Str 33, Dex 9, Con 27, Int 1, Wis 13, Cha 12
Skills: Listen +?, Spot +?
Feats: Alertness, Diehard, Endurance, Improved Bull Rush, Power Attack, Run, Toughness (3)