Any 2500 GMT400 you get will have a heavier transmission than your 1500 F44 package truck. So for automatic it'll be 4L80E vs. 4L60E, standard will be NV4500 vs. NV3500 or whatever flavor of light duty 5 speed they were using then. If automatic, you'll need the wiring harness associated with the transmission as the 4L80E has a different pinout and 2 speed sensors.
I know a guy that made a 3 door extended cab 3500HD, talk about committed. Extended cab HD's don't exist from factory to begin with. To retain his correct VIN that indicates the heavier GVWR of the HD, as well as matching his title, he cut the cabs mid-A-pillar and grafted the two together. The HD frame is completely different from all lighter GMT400's so a little bit of custom work is needed to fit the extended cab body but it's been done, just as far as I know, not the way he did it! Last I saw pictures of the project he already had the cabs joined and windshield installed. Talented man; wish I had the pics handy to share, but the forum we were discussing it on is gone (3500hd dot com)
Richard