Firstly, I made sure the intended position of the sender did not interfere with the internal baffles. This done, a 22mm hole was opened up so that the sender shaft could be passed through. Shortly afterwards, it was opened up to allow the 33mm wide float to go through too! (doh!)
The sender fixings could then be marked up.

And here's what it looks like fitted. The sender was removed again to wash out the tank of any debris. I'm going to air dry the tank for a week before installing anything back in it, as well as giving the tank a good external clean like I promised.