I don't know anything about your conditions, but I'd have to agree with casefarmer. I've spent a lot of hours in my dad's old 1480 running a 24' 810 header and I can safely say that the combine didn't want any more. It's been a couple years since I ran it in wheat, but if I remember correctly I could only go about 2.4mph in 80 bushel wheat; maybe a little faster after the sun went down. In our 35-40 bushel dryland I could maybe do 3.5mph.
Sometimes 60+ bushel per acre. Mind you, I was not breaking any speed records in those beans, but the goal was more even feeding which was definitely accomplished. In 30 - 40 bu beans I could run 4mph. And those were green stem beans. I added hardened cut-out rasp bars and the Disrupter kit which helped immensely with green stems. Really, the limiting factor was running beans over the short screens we had back then. BTW, there were very few pods in those beans!
be sure your old 1480 is up to shape put a specialty rotor in her will ahndle it easy give her a little more smoke update rotor drive to new model I have on off a 2388 laying around if someone needs one 600 # spring newer roller ramp