sanny: what braces/reinforcing is needed should be sorted by the builder...as it varies between what tubes are used.
as for top tube lengths...i suspect some of that might have been from the dim and distant past when 29"ers used the same head angles/offsets so that toe overlap was reduced. top tube lengthened out pushes the front centre out too...reducing chance of overlap.
the major differences you will generally find with 29"ers is the chainstays are longer, and you have more bb drop than on a 26"er all else being equal. this increased bb drop gives more stability in general and is why, often, they feel so ace to corner. but it also generally makes them a touch harder to wheelie/hop.
in terms of what geometry you want, it probably has a lot to do with what you want to use the bike for. ive owned various geom's and they have their strong and weak points. if you want a bike for general riding, but not too steep and gnarly, i would seriously consider giving sam a call at singular. he knows his beans and his bikes are top notch. i dont know where things stand for how much alteration can be done on the ti frames he does. be aware ti *might* end up too flexy with a kid on board as well.
if you want full bhoona custom you either need to start riding lots of them to work out what you like and take that to a builder or trust the builder to work out what you like from the things you like about current and past bikes. then if you want ti, its worth knowing a little about who is using what grade of ti and getting a feel of their skills in working the metal...again, in general, 29ers require more forming to get past tyres and chainrings if you want a decent length stay if done badly in ti, this could be a weak point or offer up minimal clearance.
as a general comment, most 29"ers these days are moving away from steep front ends and using increased fork offset to bring the trail back to a more normal number instead, making the steering less tuck prone on steep trail. downside is that if you dont get reasonably short stays you get a truck like wheelbase for a given reach....
Also, what rigid forks would folk recommend? The Surly's look great vfm but I have no idea how they ride.
