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Anyone converted a factory V8 monza from the original TH250 to a TH350? I wonder if it is just a matter of switching tailshaft to hook up the torque arm and then dealing with the vacuum modulator as the 250 does not have one

I could be wrong but the THM 250 is the same transmission as the THM 350 with fewer clutches [etc.] the THM 200 is a different transmission but I recall mine having a Modulator [that was back in 1986+/-] the H-Bodies apparently were equipped with all of these transmissions. My conversion was in a n X-body and I don't recall any strife or drama so I would think that this should be straight forward swap for you, excepting the possible irritation of speedo' gears. Harry https://en.wikipedia.org/wiki/Turbo-Hydramatic

That is correct. The TH-250 is a weaker, lighter version of the TH-350. Many of the parts are interchangeable and I am fairly sure that they both use the same rebuilt kit.

Ken