Intresting,,,,, while I can appreciate you dilema, I question the use of a salvage yard component, especially a driveshaft.
The two piesce shaft has certain advantages, and attemting to use a 1 piece unit might bring its own problems. If you go longer and one piece, the diameter has to be increased to compensate for the additional length, or vibrations will result.
How large? Who knows.
I would suggest calling a local driveshaft repair shop, and asking them what a two piece, VS one piece shaft would cost new. Most shafts start right around your budget price, and if they re-use most of your components, you might even save some bucks.
They will tell you where to measure from, and you will probably save a bunch of time and potential problems associated with using a salvage yard component.
A quality shop can take all of the guesswork out of this.
Just for the sake of arguement, the THM375B was basically a 350, same case, and considered a "heavy duty 350", while the THM 375 was a 400 case with the smaller output shaft which shares nothing with the 375B and is a light version of the 400. The 375's use a 350 yoke, so this should be a very easy swap.
Another piece of worthless trivia..............