Hey side question,
How bad of an idea is it to just replace the stem, spokes, shifters, cassette and handlebars on my bike now? Again I have a Trek 7.3FX, made of alpha black aluminum, 700x32 Bontrager Race Lite Hard Case tires and 32 spoke Nebula wheels, very similar dimensions to the 520 including chainstay dimensions, and braze-ons for front and back racks.
I know AC Mag recommended the 7.2 and 7.3 in 2008 for touring, but do you think a 4000 mile trip is pushing my luck? I'd love to do it because it would only cost about $400, 500 if I upgrade the derailleurs as well (right now Altus front, Deore rear)
No reason that couldn't work. The length of the trip isn't that relevant as long as you are comfortable on the bike IMO. I carry pretty much the same weight on a one week tour as I will for my upcoming 2500 mile tour and the same as I would if doing the Trans America again. Also I have not personally found the comfort requirements to be very much different for a 4000+ mile tour as for a organized century.
I may be wrong, but it seems like you should be able to do it for less than the prices you quote. My whole bike was $599 shipping included and I thought it was fine for the Trans America other than needing lower gearing.
I would think the Trek 7.3FX might be up to the task without many changes unless you carry a lot of weight. If you are carrying a particularly heavy load the wheels may be marginal. If you be sure they are properly trued, stress relieved, and tensioned and then pack reasonably light you would probably be fine.
I personally wouldn't use flat bars, but some riders are happy with them especially with bar ends.
If you are on a tight budget you could definitely ride the Trek 7.3FX pretty much as is. If you have more to spend a new bike is always nice
Personally once I got into the $400-500 range you mention for changes I'd start thinking hard about either backing off a bit on the changes or spending a bit more for a new bike. That is just my opinion though.