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The stock cable is far too small for the power I'm pulling for my ICE install and need to upgrade it to something suitable (read as welding cable). Would there be enough space where the existing cable is run to fit in larger or should I be looking at running a different route with my cable?
the standard alt, battery & wiring are suitably rated to each other and so the battery is going to be the limiting factor here. unless you have uprated the battery and thus the altenator then the wiring should still be suitable The stock cable is only about 8awg, and running the length of the car is nowhere near large enough when you are pulling higher currents without getting voltage drop. Upgrading the battery won't stop the voltage drop, just give a better reservoir of power once the voltage has dropped. It's surprising the difference that this upgrade will make in a normal car let alone one where the alt and battery are sepparated by such a long distance. the altenator will be rated to o/p enough current to supply the battery. the cabling will be rated to the maximum permissable o/p of the altenator. when you are drawing peak power this is supplied from the battery NOT the altenator. the amount of volt drop along that run is negligable, the volt drop that you need to keep to a minimum is from the battery to the amp etc. how much current is your ice going to draw? if your going to the point where you need to start up grading cable csa then you should start to look at the entire supply system. if your going to the stage of upping the alt wires due to volt drop over that short run then you are you taking into account the derating factors of bunching, ambient temp, retaining methods, passing through insulation, etc. as this wil all effect volt drop 
If the cable wasn't the bottleneck why would an upgrade be the first advised upgrade path before looking at uprated alternators etc. Because it makes a difference, even in a car with a couple of feet from the alt to the battery, why would IASCA and EMMA recommend it if it wasn't useful eh? across that small run will be un-noticable surely? if your really worried about volt drop then surely a cap is th way to go. the equipment that will be ran off the supply will have a wide voltage supply band, i.e 10v - 15v the alt will o/p 14.4v or something like that. plus the battery will supply the amp at what ever voltage, this will then be inverted. im not saying dont do it, its your car, but look to see if its actually required. look at the amount of current you will be drawing and remember that the stated current will be momentarily and cable rating will be for constant current draw. i have never had to uprate this cable in any of the installs that i have done. i have just spoke to a guy who works for a well know installers and he has upped the size of the alt cables in conjunction with an up rated alt and battery. The voltage drop on cable that size at the current I'm drawing is enough to be noticable on the car electrical system. Assuming the system is drawing less than a qurater of it's fused rating on music we are looking at a ~40amp draw, a bit more on transients. Along side the cars electrical requirements, IASCA recommend 4 gauge cabling as a minimum. While any voltage drop would be minimal on the sort of current the car's electrical will pull, once you are pulling more current you lose more power due to resistance of the cable and your voltage will drop at the other end of the cable.(possibly more if I actually weigh it) is one of my reasons behind driving an MR2 (then I drove one and loved it). Now while I keep umming and ahhhing about entering the competition scene I'd like to so it makes sense to me to do the job fully rather than lose points on a half ar$ed job. 
(there's actually 2 of 'em running in parrallel) run from the alt, up into the fuse box in the engine bay, where there is a join, down and along the passenger side of the car to the passenger footwell where there is another join, and then into the frunk and up to the 120A fuse inside the fusebox. Cant remember the equation off the top of my head. Haven't looked at the alt end yet tbh, was just wondering where it ran so if the sun decided to come out at some point this side of christmas (unlikely at this rate ) I could look at running a fatter cable alongside. The short path between there and the battery is a single cable. there's deffinitley 2 running in parrallel all the way to the 120A fuseable link. there's deffinitley 2 running in parrallel all the way to the 120A fuseable link. maybe it's easier of cheaper to run 2 thinner ones rater than 1 fat one Changing the cable alone will do nothing significant. Uprate your alternator and the cable at the same time, or don't bother.
