Replacing a Cat C15 with a California compliant ISX into a freightliner chassis

So here’s the back story... a company came to me asking if they could repower their frieghtliner classic that currently has a 3406 with an epa certified Cummins ISX15 with Def and all the California goodies. I said yes it was possible just cost effective hoping it would be a deterrent from the job. Well, they took the bait, fronted the cash and now here I am. I have an overhauled single cam isx with an 18 speed on the back and after treatment. The mounting of the motor, cooling system, exhaust and other component placement isn’t the issue. The issue is, can I have a fully functioning after treatment system swapped into this chassis and what kind of problems will I be running into? Please consider I have both chassis and both engines available on site. Is there any company that makes a monitoring system or harness for this particular swap and could I treat it as a stand alone engine and drive unit with after treatment?