What are you running now on your R62?. Here's just my opinion, but I think adding an air bar to either a 300 or 500 head will make them about even and double the performance over stock. Like others have said finding either in excellent condition given their age might be a problem, so you might need to figure in an overhaul.
Most 300's have two piece feather sheets, that combined with greater depth from the knife to the conveyor make them a little more difficult to feed than a 500 series. Replacing them with one piece sheets makes them a little closer to "par" with the 500's (Thank you Amos!). This all relates to beans, in wheat, etc., I don't think it's such a big issue. Like I said, I think an air bar is the great equalizer.