7500 Watt Manual Transfer Switch has #10 wires, but my well pump is wired with #8
You should be OK. The ampacity of the 8 ga is 40 amps, but who ever installed it was considering voltage drop over that long a run and used 8ga. A wise precaution and someone who actually followed installation specs...nice!
The ampacity of 10ga is 30 amps and such a short run between the generator and xfer switch won't have any appreciable voltage drop.
Most of the experienced electricians here don't really like the type of transfer switches you are using. They are over-priced for what they do and there are better alternatives, like a mechanical interlock on the main panel between the main breaker and the generator breaker. That way there is very little re-wiring to be done and you can easily pick and choose which, of all your circuits, are powered at one time simply by turning off existing individual breakers.
If you carry on with your xfer switch, there are tons of easy options to connect 8 ga with 10 ga.