Looking through my converter catalog, I don't see a listing for a 904 transmission and the 340 engine. 318"s and 360's yes, but the only listing I see for the 340 is with the 727 and the stall was around 2200-2400 rpm.
Yep, 340's came with a 727 only. So there wouldn't have been a "stock" stall for that set up.
2200-2400 sounds about right for a stock 340 converter.
Lots of people live in the 80's when it comes to converters. The newer high stall units are sometimes more efficient than a lower stall old/shelf unit. A custom converter is a good choice if you want a strong runner.