#8 copper is a codebook-legal ground for any circuit up to and including 100 amps. The only time you'd need to go bigger is if you increased the size of the hot wires to compensate for voltage drop. 3 - #2s and 1 - #8 is very common.
Pull all the wires in at the same time. This will be a fairly easy pull. You'll need one person pulling, and another feeding.
It can probably be done with a fishtape, but you might need to use pliers when it gets close to the end. Vise-grips work well.
The person feeding needs to be careful that the wire goes in smoothly, no kinks, bends, etc. They'll need to push the wire in, as you pull. After a while, you develop sort of a rhythm, and it's easier.
Lube will make it easier, but this isn't all that long of a pull, and only two 90s.
Rob