Hello,
if it ran o.k. then started to run rough I would suspect plug fouling.
due to incorrect plugs and/or rich mixture. for a few dollars you get a new baseline to start from.
if it was mine I would put a set of Champion copper core non-resistor plugs in it before anything else. modern Bosch are iffy, as are many others. and check the plug wire ends, they are individually replacable for a few $ ea. instead of buying a whole set

a '79 450 should have plain CIS with no electronic "brains". The fuel pressure should be checked at the appropriate point. Other members can probably help with specifics of pressure check. Are you sure you didn't change the control unit for the ignition instead. These are known to go bad(even supposedly rebuilt ones). Also check for a faulty coil, and plug wires. The resistor ends of the plug wires tend to go bad and cause rough running. That is where I would look first. Then I would check fuel pressure, after I made absolutly sure the ignition system was up to par first. it is possible that fuel mixture could be off also, a 3mm allen key(long) can be used at the fuel distributer to change mixture. be VERY carefull. a small turn makes for a big change across the board in fuel mixture. i think it maybe cw for rich and ccw for lean. ideally it shoulf be set by percent of CO with a exhaust gas analyzer, but it can be gotten close by ear/seat of the pants. too rich of a mixture can plug the catalyst. by the way that might be your problem(clogged cat) if car is down on power and running a little warm.
also, injectors could have a horrible spray pattern and cause low speed trouble. I would check the ignition first then do a fuel pressure check.
if it does have a ignition misfire you should know that the cat could get plugged in a hurry. I found out the hard way on my '77 450 SL I sold last year.

good luck.