I swapped one in my 70 RR last year

I bet you could do it without taking the tank down, and when you buy a new sending unit, it comes with the gasket you need

I went ahead and dropped the tank because it had a ding in it I wanted to take care of at the same time

you shouldn't need any special tools, but even if you don't drop the tank, I think you should not go about it with a full tank