1978 and up chrysler 904 lock up transmission good idea for a '70 dart build?

IMO, I think it a good idea, though, I caution you on any BIG ideas of mileage gains. A little, yes. And the low first gear is not a huge improvement, but is there one? Yes, just very small. Do also note that converters for these LU trannies are not everywhere or cheap, while not expensive, there just not a $1.99 item.

Perhaps some of the local places by you may have them as welll as junk yards. There may be a shop on line or in the back of a mag/rag that could point the way.