The D35 front axle shouldn't have a problem handling, say, a 4BT sitting over it (a 6BT will not fit between the grill and the firewall anyway, and I don't think the frame would be up for one either). Thing is, you need to use quality bearings (Timken SET-37, for example), and NEVER MIX BEARING CONES & RACES! Then set the bearing preload correctly.
Only thing I would say though is if the truck is a Supercab, I'd put the limit on max tire size at maybe 32-33" rather than the usual 35" since a 4BT is about 300lbs IIRC heavier than the 4.0L (and a 4BT's inertia doesn't dissipate near as quick should a tire get bound up). By the sound of it though I don't think the OP is attempting to build some radical trail machine.
FWIW, the outer wheel bearing on the D44 is smaller than either bearing on the D35 (inner bearing being the same size), but the D44's are spaced wider... However I've not had any issues stemming from the narrower spacing while using good quality bearings & races.