There's a shady company called portal-tek in north salt lake that was working on a reliable solution for years, my neighbor was deeply involved as their engineer but they got shady and he got out(I don't know all the details) I quit fallowing their progress when he left so I'm not sure if the got all the bugs worked out. As far as I know they had working sets, they were super spendy, I think they were starting to out source a lot of stuff to china to bring costs down? In the last few months if working there my neighbor was working on a drop out 14 bolt 3rd member that would bolt right into a 9 inch housing but I dont know if they got that done either.
Seabed like they modeled their design after the Volvo portal axles but had an extra bearing to support some gear that is common to fail in the Volvo axles.
My biggest problem with portals is cost/advantage. In order to build a rig that works well with portals you have to basically start with the portals and build a rig around them. It's pretty uncommon to be able to already have jeep/ buggy already built, slap a set of portaled axles on and have it worn as well as it did before, cog, clearancing, everything is thrown off. And IMO unless you are dodging cones in competition you probably don't NEED that much clearance and if you are dodging cones Ther is a good chance, depending on the class you are in and event, that you won't be allowed to use portal. As far as the go-fast race stuff they really aren't need but again IMO having my tires "hangdangling" off my axles seems like a weak point in the whole axle, hitting rocks at 80 is a good way to find those weak points, and I wouldn't want the end of my axle fallin off at 80, so make sure they are strong. Another problem you'll run into Is gearing, the reduction box makes for wacky gearing at the diff. Also I think I remember my neighbor complaining that they were having problems breaking outer shafts? If I remember right, he said with all the reductions they calculated that it only took 12 hp to break chromoly shafts with a 40 inch tire? I can't remember all the details but it was something weird like that.
I don't mean to discourage you, just take those thing into consideration while building yours