Difference between a 340 and 360 Torque converter please?

But not all 727 converters will interchange between the different years and "lock up transmissions" etc. The 340 converter could be higher stall BUT it MIGHT not fit into the 360 trans. We know the 360 unit is in and working so it's the easiest possibility.

Be advised that it may be necessary to use the flex plate from the 360 on the 340 to get it to mate up correctly.