There's a new wave of 29" frames rolling this way. Along come forks with more offset. The offset brings quick handling, and takes away the need for steep head tubes angles. Both improve toe clearance this way. To such an extent, top tubes can be shortened. The Soma and Niners are not yet on this new wave.
Start from here : the top tube length you prefer on 26"ers. Figure in the use of a 51mm offset fork. Fox will make them. manitou will have 47mm. Perhaps Rockshox Reba's with 51mm as well. A 71º HTA already allows for a pretty short bike now, that handles well quick enough.
Measure your current bike's front center, and decide what will be your minimum on the 29"er, corrected for the ~31.5mm larger 29"er's wheel radius.
If you're constructing something yourself, don't look TOO much at what others have done. Dare to innovate, do your thinking a little better than others, learn from their mistakes, don't just copy what seems to work. Especially as you're talking a small frame here.
Good luck,
J
