Chevy and GMC use different linkage. I have the 54 Chev with HydraMatic and Offy dual carb intake. The lever you are talking about is throttle pressure. I don't know about a progressive linkage setup. Mine opens both carbs at the same time. With the throttle pressure lever pressed rearwards and the carbs set at idle, the linkage to the carbs should just go in. The throttle pressure lever is checked with a jig but shortening the rod that goes to throttle pressure lever lengthens the shift points and speeds up the downshift poit. It also harshens up the downshifts. Too long and it will slip and run thru all 4 gears in record time. Very small changes makes a big difference. Let me know if you need more info and I will see what I can e-mail to you.
