I don't have a frame builder recommendation, but will give you one cautionary tale.
In 2001, I took off for a year of cycle touring. I'm a little smaller than you (6' 4") but myself, bike and gear was also ~350 lbs on the road. For some of the same reasons, I also thought about getting a custom bike, getting 26" wheels, etc. My departure date was May 1st and so about a year before I started looking for a custom bike to be built (to also give myself some time on a shakedown trip, etc).
I found what looked like a good custom bike builder [not any of these mentioned so far] and Memorial Day weekend 2000, I drove up to Oregon to talk with the builder. Everything looked pretty good, so I put down a deposit. It was going to take while to build and I wasn't in a huge rush and so we agreed to a Thanksgiving delivery so as to give me some time after that to do some smaller tours.
We get close to Thanksgiving and I'm told the subcontractor being used to build the frame had some difficulties. I'm willing to be flexible, and say we agree to at least get it done by Christmas. Christmas gets close and we have another delay. January delay and now it looks iffy for my March 1st departure - at least to get any riding before departure. So I decide to start off on the the touring bike I already had and then try the new one perhaps if it made sense the first part of my 2001 trip was a ride across the USA and that was followed by a circumnavigation of Australia.
The bike eventually did get delivered in late June, long after I was also in Australia. At this point I had a bike that was working for me. The new custom bike might be nice, but I also had tried and true. So, I didn't really try that new bike until after I returned from a year of touring in March 2002. Almost immediately the seat tube tears out. Back on the phone with the builder. Apparently the frame builder hadn't quite followed specifications. So, bike gets boxed up and sent back again to be reworked. It came back after that, though I never rode it much after that.
Mine is a bit longer tale of woe and some lessons I learned along the way:
- I was a little too mellow in accepting some of the setbacks along the way, and should have probably cut from the deal and asked for money back early in the process.
- Check on reputation of the frame builder (and whether they use subs and their reputations). While mine generally had good recommendations - about the same time I had problems others did as well. A well-established track record is a good thing.
- Watch your delivery dates. Custom can sometimes take a while. Prepare for backup plans including potentially doing first part of your ride on another bike if necessary.
There are things I would do differently a second time around...