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I could be wrong and if I am, please let me know. But I thought that the major difference between the 2 ODs as far as speedos go is just the drive gear where the cable attaches. One is red and the other is white. I can't remember which one goes in wich OD. But there is a tooth difference. If you change this drive, you change the speedo ratio without needing to change the entire OD."
you are mostly correct...........the blue lable O/D has a 1000tpm speedo gear setup, the black lable has a 1280tpm gear setup. Both the driven and the drive gear need to be changed....not just one. That becomes expensive and requires some disassembly of the O/D. The speedo change problem gets more complicated because, unlike every other year range, the '77-'80 speedos do not have a corresponding matching 1280 variant.
I did a '78 once and used early 70s English gauges (the 1280 speedo and matching tach). These look great in the late dash and of course are the same size as the original gauges. Change the other small gauge rings to chrome (or the large English ones to painted) so they all match and then you are all done.
