I have altered the wheelbase on a lot of trucks. The modern ones are heat treated, and we don't weld them at all. We bolt a glove on the inside and outside and put a cross member at the splice. However, trucks like yours have mild steel frames and will weld. The big concern is keeping the frame straight. We use a tape measure to make an "X" from the right front to the left rear and Left front to the right rear. We set the front and back on jack stands and level both front and back. If you put the thing together crooked, you will have a mess forever

Here are a couple of them I just recently whacked four feet out of the middle.

https://plus.google.com/u/0/1126369...7757214978&oid=112636930370126476644

https://plus.google.com/u/0/1126369...2585120658&oid=112636930370126476644

I wouldn't box a frame because they need to flex. If you box them, something is going to break elsewhere. If you have big concerns, a glove bolted inside of the frame will make it stronger than it was when it was new. Welding a diamond shaped plate inside the frame will do a lot.

If you are concerned about the welding, hire someone who does fabricating. If they do this for a living, they will know how to weld and they will know that everything has to be in square before they ever strike an arc.