258 on a flash o matic. ..

Sounds like fun. If the car is decent enough condition I wonder if the RH mounted starter style of a later 4x4 998 auto (Eagle or similar 4.2L 6 cyl 3 speed auto Jeep '80 and newer) or 32RE (3 speed auto used till 2000 in Jeep TJ Wrangler with 4.0L) or maybe even an AW4 would work with a torque tube adapter?
   Enquiring minds need a lobotomy!

The shaft is the same for TT and open drive cars, at least on the same trans and year. Americans used open driveshaft, 63 American and Classic with 196 use the same transmission and output shaft, just a different output shaft housing. I've swapped them -- both auto and manual trans models. M-3x and M-4x trannys generally use the same output housings, I have swapped an M-35 with an M-40 housing from an American (early and late), but that might vary over the years and different models.

You can adapt the torque tube to just about anything with a bit of fab work and creativity. The key is to get the universal pivot point at the right location. So a longer or shorter trans would be an issue. It's not hard to shorten or lengthen the tube itself, but the six cylinder models use a solid one piece drive shat, more like an axle shaft. THAT would be hard to alter in length. The V-8s have a tubular front section which is much easier to alter in length, but I wouldn't go more than +/- a couple inches.

I've seen a Toyota five speed with the TT adapter from the original M-35 welded to the Toyota output housing -- both were cut then welded together. On the 56-66 rear end swap page there is a photo of a crossmember that holds the TT in place in front. So it has been done... 

It's POSSIBLE to retrofit a 72+ 258 to an earlier BW trans. You will need a 67-71 V-8 BW bell housing to start with, and the flexplate. Flexplate must be neutral balanced, so remove balance weight and have checked. The crank flange will be an issue. TFs center on the crank differently than the BW. The BW uses a pilot bushing and "nose" on the converter, not sure how the TF does it, but the crank is different. BW uses a flat crank, so as long as the "nose" will fit in the end (probably need a custom pilot bushing) you should be okay. In fact, I'd drill the TF flexplate for the 258 to fit the BW flange crank. There could still be a problem with offset -- you may have to grind down the welded nuts on the converter or add spacers. You don't want the converter jammed all the way up on the pump.

As far as I know no one has done this, it's all theory. At least no one has done it that has posted on this board.