Biggest tires for mini tubbed Duster

I got a 70 Duster that's has been mini tubed and a 3" spring relocation kit
Also the inner wheel well lip has been trimmed. I plan on back halfing it later to fit some 33/21/15's and a 4 link, but for now I just want the biggest meats I can stuff under it to get this thing running.
When I was at a car show a few years ago a guy with a Purple Duster had Mickey Thompson Sportsmans 29/15.50/15 on the back, It had the 3" relocation kit and had been mini tubed. He didn't know the rim size or the offset though, but it looked KILLER!!!
This seems like it would be too tight, but It fit his car. I went back the next year with a camera and tape measure but the guy wasn't there.
The recommended tire size for the M/T's are 10-13" and the measured tire size was 12" with a sidewall width of 15.4 and the tread width of 12.9. So I'm thinking he used 10" rims to pull the sidewalls in. But would that give me enough room, remember my inner wheel lips have been trimmed also.
Any thoughts on this? They fit, I'm just trying to figure out how. Anyone else do this?