Awesome info guys. I've been reading as much as I can on the ECU logic and agree that I could just pump CNG in and the computer would compensate. I'd like to take it a step further however as I do not want to be pushing this motor.
So, my plan is to try and creat a Diesel tune that will reduce the amount of diesel given for a certain load / RPM.
I'll then create a map in my CNG ecu to replace that lost diesel. I'll also look into the timing issue and definitely try and stay on top of knock. Here's an interesting article on diesel cylinder pressure vs. CNG cylinder pressure.
http://www.scipub.org/fulltext/ajeas/ajeas21212-216.pdf