5.9 magnum flywheel for factory MPFI

I was going to run an old Mopar Perf. ECM ( I don't have the part# handy) that I bought years ago. It is OBD1 so it probably would work for what I want but I don't know yet. I figured I could carry the stock 5.9 computer as a backup just in case. If I am not happy with the performance I can always upgrade to OBD2 and then I would maybe have to find someone to tune it. It won't be a race car, I'm mostly looking for a cruiser with good performance.
Why not run OBDII out of the gate?