I get asked a lot of questions about boxing frames or not boxing frames and all the little details along the way so i'll chime in here.
Most common question is "do you have to box the whole thing?"
No you don't, however, there are a lot of caveats that go with that and it depends on a lot of other factors involved. The body itself makes the frame more rigid as well as cage work. These components will play a big part in the overall rigidity of the frame itself. The problems you'll run into relying on only these components is they ultimately leave some areas less supported than others. Now you need to figure out if those areas are allowed to flex enough to cause failure points next to where there is sufficient support. If you are starting and stopping boxed sections, there is a right and a wrong way to do it. Think fish mouth shaped cuts in the ends of the sections. Consider where the body and any cage work is attached to the frame and how it is attached. In the end, for the home fabricator its much simpler to box the entire frame an not have to worry about it. If you're already there boxing the frame, is it that much harder to finish it out, bumper to bumper. A blazer frame boxing kit weighs in around 78 lbs. Most commonly people like to "not" box the center 1/3. You're only saving 26 lbs. and its some of the cheapest weight from a COG stand point.
Probably the second most common question is "how thick should the plates be?"
We make our plates from 10ga. steel, just over 1/8" thick. Some have requested 3/16" or 7ga. steel. Obviously 1/2" thick would be retarded as would 18ga. but realistically its the shape of the completed structural member that is most important here. The rectangular tube wins the day. We can just as easily make them from 7ga. but you're paying for shipping charges that really don't do you much good. Going up from 10 ga. to 7 ga. increased the weight of the kit from 78 lbs to 117. Again, not astronomical but its an additional 39lbs that is IMHO really unnecessary. The only time I can think of that a thicker plate may be better is if you're dropping your chassis on the rocks and want the extra thickness to prevent some denting. There are better ways to protect those vulnerable areas, like skid plates and the like,.... and there is another thing that may increase the rigidity of your frame.
Another common one, "plates on the surface or tucked back inside just a touch?"
Structurally it doesn't make a huge difference. Easier to weld tucked back inside a touch, however, you must now consider things like cross members. If you want them to simply unbolt and drop straight down, that lip will make a difference. Easy enough to work around. Let's suppose you leave a 1/4" lip. Use a 1/4" plate where that cross member is placed to effectively eliminate the lip and the cross member will drop straight down and free from the chassis. I guess on this one, consider your welding skills and do it accordingly. Personally I think it looks much cleaner to put the boxing sections on the surface rather than slightly inside but that's just me.
Other things to consider,
The front shackle hanger will need to be dealt with. A tube welded through both sides of the frame will do the trick, just keep your frame thickness in mind here.
Steering box bolts, Same solution is the best. Drill straight through both sides and use a bushing welded both sides. DRILL STRAIGHT!! take the time and build a drill guide.
How to attach your cross members should be planned ahead. Some should definitely be removable. Others not as important. Some I like to make a generic flange on the frame. Then the cross member itself can be built for a specific item and changed later for a different thing. Think engine swaps. One flange that could work for several cross members.
Hanging items such as fuel tanks and the like should also be planned ahead of time.
Last one, "is it worth it?"
I can only answer this with my opinion so here goes, Absolutely! After having wheeled a truck before and after boxing the frame with no other real changes, the difference was night and day. The steering box that kept coming lose, stopped coming loose. The creaks and moans and groans coming from the chassis and body were completely eliminated. For the first time we got to see what the suspension was actually doing rather than a combination of the suspension and frame flex. The doors opened no matter how twisted we got things.
One last word, If you're not familiar with working sheet metal, get some help. I'd rather you not buy our kit then purchase it and do an install that you are unhappy with and potentially ruin your frame. The kits come in flat form in section lengths determined by what cuts well for us and ships well through UPS. This does not mean they need to be installed this way. You may want to cut them down, you may not. You need to be able to bend the material in and out around the various curves of the frame. As stated above, none of our frames are the same as they were the day they came off the assembly line. We've gotten the box kits close but they will require fitting. The kits are designed to be a convenience. If you have modified your frame extensively or you are trying to modify a kit to fit something else, the convenience may have just flown out the window.
If anybody has any questions I haven't covered, ask away or call me. Always happy to discuss.