8 gauge - NO BLOODY WAY. Only rated for 30 to 55 amps depending on specific wire type - and in a hot mining environment you need to DERATE for temperature.
6 gauge - probably OK depending on the wire type.
http://www.usawire-cable.com/pdfs/nec%20ampacities.pdf
Thickness of the jacket is actually a MINUS for it's ability to carry current without overheating, as the jacket is an insulator that RETAINS heat.
DO NOT WIRE BREAKERS UP IN PARALLEL UNDER ANY CIRCUMSTANCES.
THAT IS AN EXTREMELY UNSAFE CONDITION.
220 does not "need" neutral to feed pure 220 volt devices - the only time a 220 circuit uses a "neutral" is if it is ALSO feeding 110 volt devices, or if it is feeding "dual voltage" devices like a lot of ranges that use 110 for the clock/timer stuff.
A breaker on a circuit should ALWAYS have the same or LOWER amperage rating of the WIRE used on a circuit - this makes the BREAKER the point that will "fail" first.
Doing it the other way around is a major safety hazard, as it can allow the WIRE to overheat and fail before the breaker pops open.