I've seen 6"-8" pipe with holes torched in it successfully used to distribute heat/smoke in a traditional offset and see no reason why it would not work in a reverse flow. I built mine with tuning plates sitting on 1" angle tacked to the inside wall. Here is a link showing & explaining mine:
Buttburner posted a better picture, same basic idea.
I think you could adapt this concept into 2-3 1/4" plates you could work in around the existing fabrication and weld in place or just let gravity do its thing. IMHO reverse flow, however you get there is the way to go.
If you already have pipe its probably worth a shot, but I'm a little concerned about getting enough flow with one 6" pipe, can you fit 2?