I hate to differ with gggGary, and I really hate to differ with 5twins, especially when it comes to carbs, but 78-79 uses the "new style" slide, and 70-77 uses the "old style" slide, and the springs are different. At rest, the diaphragm spring for 70-77 is approximately 4-5/8' long, while the 78-79 diaphragm spring is approximately 4-1/8" long at rest.
The parts fiches show 256-14933-00-00 for 70-75, 584-14933-00-00 for 76-77, and 2F0-14933-00-00 for 78-79.
EDIT: Okay I just read the whole thread.
Perhaps there is a third length, the 4-3/4" that dirtymike measured, which would be for either 70-75 or 76-77. Anyways, in this case, 5twins was right, as usual, gggGary too, just use a matched pair.
Sorry for the huge time waste, but I have to admit that I've always been a bit obsessed with the different spring lengths...