Cut and weld the frame? The drivers side front axle mounting bracket needs to welded on, if it isn't on there already (some 2wd trackicks already have the bracket). Other than that, zero other frame mods are necessary...and definitely no cutting of the frame is necessary.
You really don't need a whole donor vehicle, but it may be cheaper to find a wrecked/rusted one cheaper than buying parts piece by piece. You'll need...Complete front axle assembly with gear ratio that matches the existing rear axle ratio, front spindles, lock out hubs (auto hubs will work too), driveshafts, tranny/tcase crossmember, 4wd tranny, tcase, speedo cable (the 2wd one will be too short to reach the tcase).
I have done a few 2wd to 4wd conversions and I have either posted on here a pretty detailed write-up, or I have linked my write-up I have in the How-to section of zuwharrie.com on 2wd to 4wd conversions for trackicks.
Other than possibly having to weld the one bracket on the frame, it is really a pretty easy, remove 2wd parts-bolt 4wd parts on, modification.