per the supplied part number, each relay in the MSD pack will handle 40 amps which "should" be adequate for the horns unless you have some big monsters on the vehicle.
The rest of the items you mentioned should be verified to be within the 40 amp spec. Myself if it's rated for 40, first question is that continuous or intermittent.
Obviously the horn would be intermittent.
Something like the fan should be considered continuous as would the A/C headlights etc.

Myself, if the relay is rated for 40A continuous I reduce that number by 20% or so for a "safety net". The cables/ wires should be sized depending on the distance from the power source.
If all four circuits draw 120+ amps continuous ( unlikely) you'll need a 4 ga or so cable if your within 5 or so feet of the power source. I would also encourage the use of fuse(s) as close to the power source as possible.