Are you sure it has 6' offset?  I have a 36' 974 Macdon and if I remember right I used 13" as the offset, which should be half of the total offset which would have been 26".  

You need to enter half your total offset to center the head.  Also I'm not sure how it is worded in the EZ500, but right or left can mean implement or antenna.  So if it doesn't work the one way try the other.  I use a Raven cruizer with Smartsteer and once I got the offsets figured out it works awesome.

another thought: I took a tape measure and measured from the end of the header to the antenna on both sides, then figured out the difference.  That way it is exactly right to the inch.