The emissions equipment in and of itself will not likely cause you to not be able to maintain 55. I'd first check to make sure your compression is up to specs. My truck runs decent but spews oil smoke out of the dipstick hole. Minimum spec is around 100 PSI, mine is 75-85 PSI. This is enough to cause low power, hard starting, etc.
If you are not using oil, and you have no obvious mechanical problems (also remember the valves are manually adjusted, and also a check of the timing chain, by using the distributor), I'd look to fuel and timing. This is where many people toss the emissions equipment and I can't blame them as the system is hard to diagnose and relatively complicated, and parts are not readily available (new). Not saying you can't make it work perfectly, but if its a driver you have very little time to diagnose.
Most people in this boat convert to a 70's electronic distributor and a same-era Duraspark box, as well as a Motorcraft non-feedback carburetor. Please note that you can take the feedback carb out of the loop and convert it to non-feedback or keep the EEC IV ignition and use a standard carb. This will eliminate some wiring and hoses.
Personally, the very best setup for a carb'd 2.8L would be a remanufactured (a GOOD one... not a junk piece) Motorcraft carb tuned with a wideband and using a modern high flow catalytic converter that does not require air injection. For ignition I would use a brand new Duraspark and new 2.8L aftermarket distributor, and possibly replace the Duraspark with an MSD / Crane / Mallory ignition box.