[Joe K]

Likely you would have the same issue with a replacement - or maybe the converse - shaft too short resulting in too much forward-back movement of the shaft/impeller. Sellers generally sell what will cover most circumstances and too long is better than too short in that instance.

Your thought to measure an existing functional pump is a good one. Just leave yourself enough to remove with the grinder so you don't "overshoot."

Overshooting can result in a need to "add" metal, or do the "tap & insert for a brass bolt" extension fix. The fix can be performed by having a welder add metal and re-grind.

My measure might be done using a technique called a "bridge gauge" - basically amounting to three blocks of wood set up to "bridge" the impeller. But I have the vernier caliper (with extension) to measure how much bridge the bridge actually bridges. A ruler might work if you're careful.

Joe K