340 and 904 trans

Do you mean they're phasing out converter manufacturing for older 727's, 904's, T350's, etc, etc.??? Are the only ones you can buy from Summit, TCS, etc rebuilt ones?

Yup, new only from Car makers, and only for current production transmissions.
Not a $$$$ wise deal for them to make new units for 40 year old cars, or anything not current. I worked for a Chrysler dealer back in the day, and on the box a converter would be in would be "rebuilt for Chrysler", and when I worked for a GM dealer the converters came in a box with either the letter R in front or behind the part number. The R stood for rebuilt. Summit, Jegs, etc. buy from rebuilders, TCS rebuilds their own.