If you want to increase the 1-2 upshift speed, you can do it without removing the pan.
If you have a carb with large throttle bores, it requires less throttle opening than a carb with small t/bores....to get to the same road speed [ shift speed ]. The K/D lever moves less with a big carb, all else being equal; & the trans shifts earlier, all else being equal.
The above demonstrates that it is a 'ratio thing '.
If you want later shifting: there is a rod from the carb that goes to a bracket/linkage at the back of the engine. Where this rod hooks onto the bracket, the hole or stud on that bracket needs to be repositioned closer to the pivot.
The idea is: for the same throttle open position, the K/D lever moves further, increasing line pressure & delaying the shift point. You might need to experiment with different hole positions. The same thing can be achieved with the K/D lever on the trans; move the linkake hole closer to the pivot.