Shortening the shaft can be somewhat easier or a lot of work. To find out, you need to use a press to lift the hub up and look at the shaft. There were some pumps which used a stepped shaft...meaning you just pressed the hub on as far as it would go and it was automatically at the right spot when it bottomed against the step.
I wasn't aware there were different types. Just so I understand this correctly, you're saying I have to press the hub a bit as though I'm removing it, and then look to see if there is a step where it was located? Sounds easy enough.
I'd like to just make my own short shaft pump and try that first if I can. If it overheats, then think about the adapter plate. Beats buying a short shaft pump, spending the money, and then ditching it later.
