Dietrich Go to You Tube and enter "Ribbon Burner" in the box. Lots of information. Over the years of seeing other shops I have seen two other approaches used to heat longer lengths of steel somewhat uniformly. A longer gas forge with several burners spread out over the length is one. A lesser way was to have a long forge with a SINGLE LARGE BURNER and just take a longer time for the whole forge to heat up for its length. I don't think this last would work for great lengths, but if I recall I saw it used for a forge that was about 24 inches long. The forge was about that long with a small cross section and the burner was aimed into the front door of the forge(not down into it from above).
It was a while ago, but I think it was at Andrew Crawford's shop. I believe that the back of the narrow forge body was pretty much open to allow the heat to move to the rear. The cross section of the forge was quite small and I definitely think it was designed for heating bars over a longer length. I think Tom Sheets could be of help. John

Added 52 minutes later:

Dietrich I may have erred in assuming what you were trying to do. I was thinking in terms of long uniform heats. The You Tube videos show more of what you might want. John