Normal household cable will not carry the amperage without resistance and heating up I would go with something like this:
http://www.vehicle-wiring-products.e...85018069034630
You might get away with 39 amp as opposed to 50 amp. If you are wiring right back to the alternator stator you will need to check that the heavier wire will go through the gland in the alternator cover.
Measure the OD of the existing cables and compare with the specification of the various thin wall cables and use the highest amperage cable that will go through the gland