I agree with the above two statements. The TH400 is stronger out of the box, but a well built TH350 will be just as good, and the gear ratios are better on the TH350 than on the TH400. That plus less rotating mass will result in better all around performance, and less fuel usage due to power loss in the transmission.
One other aspect - unless you get your hands on an Oldsmobile "TH-375", you will be replacing the yoke on the driveshaft. In either case, you will be shortening the driveshaft, and relocating and potentially replacing the transmission xmember.
A lot of work for less 'wheel feel' performance.
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1988 9C1 - Modified LM1 @ 275HP/350TQ - TH700R4 - 3.08 8.5" Disc Rear - see it at
http://www.silicon212.org/9c1!
2005 Crown Vic P71 - former AZ DPS - 4.6 liters of pure creamy slothness!
1967 El Camino L79/M20 old school asphalt raper
Remember - a government that is strong enough to give you everything you need, is also strong enough to take everything you have.