8 AWG is good for 40-55A depending on insulation class. What you are doing looks fine to me. I might upsize the main feed to 4AWG so you'd have plenty of capacity there. Also you might want to supply your fan relays directly from the battery instead of putting them on the end of the chain. They're going to draw 10-15A apiece. The way you have it drawn out, there are going to be lots of loads on that one 8 AWG cable so you don't want to have the fans taking up 50-75% of your circuit capacity. I'm also assuming where you have written in "Starter Solenoid" that you meant to write "Alternator", correct?
All you have to do is get the right relay. They are usually readily available in 30A, 40A, and 60A. I would get the largest relay and then fuse the supply before the relay so you won't have to worry about any slim possibility of burnt contacts if your relay is right at your main circuit draw capacity. Fuse the supply side of the relay based on your cable size. I see you are going to do it with fusible link. I personally got rid of all the fusible link and replaced it with fuses.
Here's what I did. It is similar to what you are doing but without some of the junction points which as we know just causes more chances for problems and resistance. Also, I'm using ATC fuse blocks instead of terminal blocks. They are readily available at most parts stores.
I'm using a 105A one wire alternator and am using the original harness to supply the car's fuse panel except I got rid of the main fusible link and replaced it with an auto reset circuit breaker. My auxiliary circuits are used to supply anything that did not come on the car originally and they are fed by new wiring that is separate from the main car harness.
Ran an AGU fused cable straight from battery positive into the car and under the console.
Connected to main feed on a six circuit auxiliary fuse block that can use up to 30A fuses on each circuit. They have a male spade on them that is only hot when a fuse is inserted.
That's the "always hot" block.
Fused one of those outputs with a 30A fuse, ran it across a 40A relay, and to another auxiliary fuse block also under the console.
The relay is activated by a wire run from the ACC terminal on the main fuse panel.
So, now there is a six circuit always hot fuse block and a six circuit keyed power fuse block.
This is what the fuse blocks look like. They're made by Cooper Bussman and are a little bigger than a credit card.
Here is an AGU fuse holder.
These battery terminal extenders are good too.