What size would be good for 8.25/8.38. Should I go 5.6 or 5.8?
Both those two above have it right.
If you are more a street tech, the 5.6 will work better on both the 8.25 and 8.38, but if you are more transition based or like a wider more stable ride, the 5.8 is more the one and is good on 8.38 more than 8.25 sized boards.
There are two guys I know and skate with who are very much those exact types I mentioned and riding those trucks as said, both finding they are perfect for what they want.
Putting three washers on the inside of 5.6 and having one washer on the inside of 5.8 are quite close, but the 5.8 will stick out more in the axle ends with that in mind.