Evidently Pontiac thought that a T400 was necessary as that's what came in the car from the factory. As with any car company, they get away the cheapest way possible and they only used T 400s in the 400/428/455 GPs. They could have used a T 350, but didn't believe it would make it through the 5 year 50,000 mile warranty.
The 4L80E is a direct decedent of the T 400 and even uses many of the T 400 internal parts. The basic differences are the OD unit added to the rear of the 400 and it is electronically controlled rather than mechanically controlled by a governor and a vacuum modulator to determine shift points.
If you want to be freshening your transmission periodically then the 200/700 is for you. If you want a design that you can depend on without periodically freshening it then the 4L80 is what you want. Don't forget the large initial outlay for all the good hard parts to replace the weak 200/700 stock parts in the beginning also.
I personally own a 5300# 93 K3500 dually with a 6.5 turbo diesel that was a construction company truck before I bought it and it wasn't babied, 237,000 miles pulling, hauling, and plowing and the 4L80E is still doing it's job just fine. The diesel has a torque rating of 440 lb.ft. enough torque to break a weak transmission. The turbo diesel and BB chevy is the reason for GM designing the 4L80E in the first place, they didn't band aid a 200/700 to put in their trucks behind a higher torque engines because they knew it wouldn't make it out of warranty under any kind of hard usage.
It will tow my 27 foot 7000# fifth wheel camper to Norwalk in the next 2 weeks, that's just over 6 tons total weight between truck and trailer. When a stock 200/700 will last that long getting used hard, let me know. Even a beefed up one won't last over 200,000 miles under those conditions.
The whole crux of the matter is why put a transmission that was never designed to handle the torque of a Pontiac, stock or modified in the first place. Transmissions that were designed for V6s and small block 305 and Olds 307s and even broke under 50,000 miles in moderate usage not being abused. Pick your poison, but choose wisely because after the weaker alternative breaks the first time you'll have twice as much money, and time in your bargain getting the car back on the road again.
Remember the 63 SD Tempests with the hybrid clutch 2 speed auto, never designed to handle the power of a 421 SD engine, originally designed for Corvairs and 4 cyl Tempests. Pontiac did their best to beef the Tempest driveline however it would need freshening or replacement after about 10 runs down the 1/4 mile. Racers quickly realized they had to look to a tougher package to continue drag racing the SD Tempests and converted to a standard rear axle with a transmission behind the engine. Torque breaks inferior parts not designed to handle the power, and it did in this case too.
Hammering the throttle of your HPP and not worrying about breaking the transmission, or freshening it like a wear item, such as brake linings.........................Priceless......... ..
