If you adjust the pump stroke as to specifications,the minimal difference in length really doesn't matter.If it really bothers you,on some but not all,you can increase the over all length of the pump.Put the top barrel in a vise and use a small pair if diagonals to pry the shaft out,you may or may not get to the length you need but doesn't cost anything to try.

I am confused why trying to raise idle with front carb kills engine. Pump is a separate issue with accelerating, and should not have any effect on idle. Frankly, getting a good idle is mostly the rear carb. Your just trying to ensure you have mixture right on the front, and then open throttle blades with idle speed screw to gain last 50 or so rpm you want. If opening front primary throttle blades cause the car to stall, some thing not right, and I think would have to be way off.

Dragon Slayer, I am confused too. What controls the idle? I was getting no gas coming out of the two nozzles inside the carb when I activated the throttle. I assumed it was the acc. pump. The rear carb is providing 95% of the idle and I was trying to get the front carb to contribute the other 5% when I increased the idle screw.

Lets be clear. The idle mixture screws, which are the hidden screws on the front carb control fuel to the idle circuit that already has some fuel and air coming in through an idle port on the carb body. Screwing in the idle mixture reduces extra fuel and leans out the idle mixture. Screwing out richens mixture. Your adjusting the screws front and rear carb to get best idle and vacuum. Then turn the actual idle speed screw which pushes on the throttle shaft lever to open or close the primary throttle blades. Normally probably coming out to get idle back down around 650 to 700 or what ever your manual calls for and what you want it to idle at. Then do the idle mixture again to refine mixture. Then adjust idle speed back down with lever screw. Normally this is done with progressive linkage detached. And front carb idle screw backed off and throttle blades closed. The rear carb should idle the motor. If you want 650 idle and the mixture screws are all good. Get 600 on back carb, then adjust front idle speed screw to get 650.

Reattach progressive linkage and test.

Important point here. If those screws are really loose, and if you have lock screws they have to be in place though not tightened down. You need to ensure no air leaks coming in around mixture screws or locks that can be upsetting calibration.

Also make sure good ignition set up and timing first.

Now if your idling car on rear, and just open front with rear at idle, maybe it will stall. Never tried that on car. The front does not open until the rear is well past mid throttle with the progressive linkage.

Gas flows through idle ports, a full depression of gas pedal would pump fuel out of nozzles, but that would assist with starting, and is really the acceleration circuit to shoot fuel as the carb transitions between low speed and high speed circuit. Since you don't seem to have any problem starting car, it should idle and accelerate to moderate speeds in neutral without the pump shot.