yes, you can build a D44 out of a bare housing with D30 parts. I've even seen 9" high pinion fronts with D30 parts. when you buy a bare housing, you MUST get the matched bearing caps on the D44, otherwise its a paperweight. i've thought of doing a HP D44 with junkyard D30 parts, so I could use Rubicon D44 shafts, still retain my Vanco brakes, and get 5.13 or 5.38 gears.
basically you cut the D30 knuckles off
and its a good time to upgrade to 3/8" wall DOM tubing instead of the factory 1/4" wall. this requires machining the ends down to 1/4" wall.
basically press tube into the D44 housing, press the knuckles on and weld them where you want. you set caster vs pinion angle here. you need to have alignment equipment and/or the knowledge on how to assemble the housing so the camber & caster come out properly. you set the length such that the aftermarket or standard shafts work. when welding to the cast iron pumpkin, you need to use high nickle rod. obviously you can really screw stuff up here - not for the backyard shade tree mechanic.
lastly, you weld the coil & control arm brackets on in a position of appropriate caster, pinion angle and get everything lined up at full bump...shock length, bumpstops, etc.
here's the build thread I took those pics from:
Yellow, Red, Long and Soft.... - JeepForum.com
As you can imagine, it's not cheap to have someone build this properly. Its several hours of labor, and the guys that know how to do this ain't cheap.