Man, the Bronco ain't even a work truck - Ford didn't care enough to install a SEFI on these things, what makes you think they put in an HO engine? Being an '88 you should have the HO heads, I give you that, but no way you got a roller cam and forged pistons, not from the factory at least. The firing order is based on the camshaft, so if one of the previous owners installed an HO or a 351 cam in it that will require the firing order to be changed... with a SEFI system you'd also have to re-pin the injector harness, but you got batch-fire so that ain't no issue for you. With the fullsize cars we do all sorts of HO swaps, from just intakes to full HO swaps and everything in-between - I know of a guy who runs a ported non-HO intake and EGR spacer on an otherwise HO engine cause it gives him more air speed and better fuel economy and somewhat more low-end torque. Like I said, unless your 302 came from a Mustang or a Mark VII LSC you ain't got no HO engine, just an HO/351 cam.
Now, about timing, don't even try to measure what the ECM is doing with a timing light - how much advance it pulls depends on many things you really have no control over, such as the ambient air temperature for instance, the quality of gas, and so on. If you throw the light on it while idling fully warmed up you only see what tis doing at that particular moment for those particular conditions, bet if you somehow got a full tank of summer-blend gasoline now and ran off that you'd be seeing different things. So just cause most folks say their ECMs pull 20 degrees of advance at idle don't mean yours will do the same at the exact moment you decided to take measurements, and you having that cam in there throws things off even more.
By the way, I ain't bashing on ya, I'm just trying to explain some misunderstandings. However, something that just came to me - you said your truck runs beautifully at 20* initial, and since that clearly ain't the stock cam, what if during the install someone messed it up, and ended up retarding it? Most timing sets allow you move the cam 8 degrees in either way, so if the installer wasn't paying attention they could have ended up with 8 degrees in the wrong direction, which certainly affects idle and low end.