Unless you are going to add some serious power, (like over 350 Horses), a truck frame is heavy enough that it shouldn't need boxing. That being said, here is what you do. First, get some heavy cardboard, (not the corrugated stuff boxes are made from), it is available in 24 x 36" sheets. measure the widest (deepest) part of your frame, add an inch, and cut the cardboard into strips that width x 36" long. You could conceivably glue all the strips together to make a peice of cardboard as long as your frame, but it really isn't neccessary. Starting at the end of the frame where the boxing starts, hold the cardboard against the inside of the frame and trace the outline of the frame on the cardboard. Measure the thickness of the frame and offset the traced lines towards the inside of the pattern by that amount from each side, and cut it out. Repeat this over the full length of the frame, keeping in mind that the ends of your 36" peices of boxing plate will have to butt up against one and other neatly for welding. Go to your local welding shop and buy some 1/8" (approximately #11 gauge) hot rolled mild steel plate. You can buy any width you want, but this stuff weighs in at 5 pounds per square foot, so a 36" x 72" peice weighs 90 pounds. Trace the patterns you made out onto the steel plate with a soapstone, and flame cut them with oxy acetylene, plasma, or even a saber saw with metal cutting blade. Bevel the edges on the side which is going to face away from the framerail at 45 degrees with your grinder, all around. The next step is a matter of personal taste. You can inset the peices of plate into the channel which forms the framerail untill the boxing plate is flush with the frame, and weld it like that (that looks best) or you can steal a page from "Pete and Jake" and set the plate in an additional 5/8" farther into the c shape of the frame, and weld it like that. That is called "step boxing", and gives a kind of "ledge" on the inside of the frame on which you can route wiring or brake lines, But it doesn't look as nice. Make sure that your frame is perfectly level, and if you have the body off, check cross corner dimensions to make sure it hasen't parallelogrammed. You can use a mig here, as long as it is capable of welding 1/8" plate (check the manufacturers recomendations). Tack everything into place using light tacks, then weld only about a maximum of 2" in any one place, then jump to another part of the frame and repeat the 2" weld. The reasoning here is to avoid any undue heat build-up in any one area, which could cause warpage and misalignment problems. When all your seamz are fully welded, a bit of work with the grinder and some 36 grit discs will clean it up.----a note here---if you "step box" the frame, you don't have to grind the 45 degree bevel on your boxing plates, and there is very little grinder work afterwards. However, if you keep the boxing plates flush with the outside of the frame, you must grind the 45 degree bevel on the side of the boxing plates which faces away from the frame, so that when you grind everything to make it look good you will still have sufficient weld to hold everything together properly.
I hope this helps, and keep in mind that this only applies to frames which are formed like a c-channel.