I run a flipped 300 with 4:1 and have upgraded the output. No complaints yet, but by the time you do all of that you can be close to the cost of an Atlas. I did mine in parts so it was easier for me to afford. If I had to do it all over again, I would choose an Atlas....
if you're gonna do that , you could just either A: buy a 4:1 kit for your 231. B: find a 241OR off a rubi and swap it, or C: buy a 300, flip it, buy 4:1 , and make shifters and stuff, or D: buy an atlas. id wheel for a while and look for a 241OR personally. the 300flip/231 doubler adds up fast, or even just a flipped dana 300. i researched this for days a while ago. i decided to keep the 231 until i can afford an atlas.
i had a flipped 300 & 231 doubler in my heep.
i snapped the output shaft the 3rd time out on the trail.
i wouldn't put any money into upgrading a 300, but if you can get one complete they work pretty good until you go with big tires (i had 39.5 iroks & 5.38's) or heavy skinny pedal use. just get an atlas & save a ton of money & headaches over the long haul.