Probably wants the width of the D44.
The D44 actually has narrower (closer together) mounting points than the D35, however a D44's pivots can be bolted directly into the space where a D35 was. You will have to shorten the passengerside axle shaft(s) a total of 1.5-2" to account for that difference in pivot spacing (or better, you could instead lengthen each beam 1", which would preserve the axle's entire original width).
If you're putting a TTB where one did not exist before though (Jeep, etc.), then yes your brackets will be pretty much entirely custom. If you don't have any references to measure from, I would set the spacing of both sides so that the center axle slip-shaft is about 1.5 - 2" from bottomed out with the suspension positioned at normal static ride height.