OK.
The stock alternator (140A) should handle your additional load level.
I'd use 10AWG, stranded (not solid core) which is good for 12VDC up to 40A with a 3% line loss, at 1M in length.
Under hood space is limited, especially around the battery. Wherever you put the extra fuse box, etc., be sure to secure it well to avoid vibration. I'd put the relay inside that box as well (also secured). I'd still put the 40A fuse and its holder as close to the battery as possible.
And again, however you terminate your wires (screw-ons, bayonet, socket, other types of push-ons or direct connections), be sure to solder those connections to the wire(s). Very small contact resistances can play havoc with maintaining proper voltage levels and, as a result, proper power transfer and the introduction of connection hot spots.