First, good luck with your project.
Regarding your welding question . . .
If you mean cutting and extending the frame, then welding is the only way. If you are in a large city, there are certainly frame shops that can do that for you. The price here in Grand rapids, MI is currently about $1800.
I had my frame extended six feet in the back after I dropped the front driver and extended the driveline. This was to accomodate a 16' box I was putting on it to make a motorhome. The frame was welded, and has held up nicely.
Now, if you mean welding "to the frame" to mount brackets for side boxes, or battery boxes, or for whatever, there is a school of thought that says "no, don't weld to the frame." Further, the Federal Motor Carrier Safety Regulations Handbook implies
it is prohibited (393.201), saying all accessories attached to the frame must be bolted or rivited.
That said . . . your vehicle is not in commercial service, so technically all that federal stuff doesn't apply. You can't be put "out of service" for welding to the frame because you are not "in service." So do what you want, it's your truck. Just be safe.
I have three large stainless steel "tool boxes" installed on my "motorhome" used for storage. The brackets were all welded to the frame, as were the mud-flap supports. No problemo, so far.
About the only real caution I will pass along regarding working on the frame, is DO NOT DRILL, WELD, OR IN ANY WAY ALTER THE TOP AND BOTTOM FLANGES OF THE FRAME RAILS. This is where all the strength comes from, so don't mess with the flanges.
I will be excited to hear how your progress progresses.
Keep us all posted!