I wouldn't think that adding a Cat/DOC/DPF to a 80x set would be a good idea. These sets were design for the military and they are generally exempt from most emission/pollution controls for the sake of reliability. A cat/doc/dpf needs a certain temperature range to work effectively.Too cold and the catalyst won't activate,too hot and they melt. Plus the additional cost of the components needed for the system to work properly would be very expensive for a retrofit. and not to mention the packaging requirements for those components.
I did a quick lookup on a Tier 4 emission 4 cylinder diesel engine. Just the DPF for one ranges from $1300 to +$2400 new. And i don't think that includes the DOC, which can range from $500 to $1200. Then add the cost of the necessary ECU controls to make sure the system runs properly...and its honesty not worth it in the end.