After conversations and email exchanges with vendors and ultimately KYB tech support...I installed the KYB Excel-G 344492. The Bilsteins were too short and could risk some damage due to a shorter stroke than the OEM. After reviewing the available data for the rear shocks of the M45, I found that the KYB 344492 for the 2004-06 Infiniti G35 AWD had very close specs to the available Monroe shock for the 03 M:
Extended Length - 22.12 inches
Compressed Length - 14.8 inches
Stroke - 7.32 inches.
Extended Length - 22.140 inches
Compressed Length -15.210 inches
Stroke - 6.930 inches.
My only concern afterwards was damping. This would be trial and error at my own risk. When I compared the KYB to the OEMs, aside from the specs, the KYB had a larger diameter tube. Well, after installation and about 500 miles later, the car handles much better and has a firm ride. No regrets or issues so far. I will say the car now has about 167K on it and it was riding on the original shocks. Which according to the Infiniti dealer...were still in good condition.
My next effort will be the fronts. These will be OEM, yet looks like the best price is http://discountinfinitiparts.com
Hope this helps someone.