Ranger/B-series frames come in 2 parts, front and rear, joined together just behind the cab
Rear only came in 2 sizes, 6ft bed and 7ft bed
All were 6ft beds except a regular cab "Long Ranger" which had 7ft bed frame
As said all extended cabs have 6ft beds
In 1998 the 4WDs got a new front axle and new suspension, so for your 1999, a 1998-2011 extended cab 4WD front frame would be best match, from Ranger or B-series
There are 6 different front sections:
Torsion bar Supercab cab 6 ft bed
Coil spring Supercab 6 ft bed
Torsion bar regular cab 6 ft bed
Coil spring regular cab 6 ft bed
Torsion bar regular cab 7 ft bed
Coil spring regular cab 7 ft bed
The coil spring are all 2wds, the torsion bar can be either, although I don't know if they ever made a 7ft 2wd - I've owned one of everything else.
The 7ft bed front sections are 6" longer than the 6ft bed ones from back of cab to the separation joint.
For reference, the 7ft bed rear frame is 6" longer from the rear spring mounts to the back end of the frame.
If it was the rear section of your truck that was rotted out, it is just a "lot of work" to replace that section of frame. And individual doesn't need special tools (I don't consider grinder or jack stands to be special tools).
If you are replacing the front section, it is a "#@$% of a lot of work". And you are going to need a lift to remove the cab from existing truck to the new frame. You will also need the lift to remove the engine/transmission/transfer case to swap it from frame to frame.
It's been done, but usually the truck has sentimental value e.g. Grandpa's old truck.
If you were here in Calgary, I'd gladly swap you time on my Ranger for time on yours; I might even lend you some tools but western MI is a little far.
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