Best size for J-tube for homebrewing application

I am a home brewer and would like to fabricate a very simple outdoor Rocket stove with a cast J-tube design. I will probably cast it with a clay and perlite mix. My question is, if I will be boiling about 8 gallons of wort (pre-beer), what size system will be needed to bring this to a full boil from about a starting temperature of 160 degrees F?
Keep in mind, I would like to bring it to a boil in under 30 minutes.

I was considering a 6" round J-tube with a 12" feed tube, 12" burn tunnel and a 30" to 36" heat riser. The pot with the liquid would be over the heat riser just a few inches.
Also. I would probably have to bury the feed tube and burn tunnel underground so that the cooking surface won't be too high as the pot weighs about 80 lbs.

So what do you think, will the 6" system be adequate or should I opt for a larger 8" system?

Kirk Leidemann : At the top of the page locate the Permies [Search] Button, clicking that will take you to a new page where you can enter " RMHs'' +'' Maple Syrup''

In the Search field, then select a Google Search within Permies I got forty 'hits' for this related subject ! Good luck! Good Hunting ! For the Good of the crafts ! Big AL