You can also go with the 200R4 behind the 250. It's not as strong as the 700R4, but more than adequate for the 250. I would advise against the TH400, it robs several horses. The TH350 would be a better choice for a 3 speed behind your engine.
Some people swear by the Powerglide, which was available in 68. I had experience with the back then, the only one I had success with was in a Nova behind a 194. It is another option.
