I used to work at Monroe Truck Inc, they used to stretch a lot of big trucks for industry.
All of the trucks I saw they stretched had "C" channels and at least 1 crossmember added to the original frame. There was a 8" to 12" overlap (probably depending on the expected load the truck would see) on both ends to the section that was added, always ahead of the rear axle. The C channels were bolted to the original frames with 8 bolts on each rail, at each end of the added section. There were 2 sets of bolts on the side rails about 4" apart (depending on the amount of overlap), and 2 bolts on the top and 2 bolts on the bottom flanges, also about 4" apart. These grade 8 bolts were probably 3/4" or 1" diameter each. Each section added had at least 1 crossmember (most had 2 crossmembers) with a drive shaft Currier bearing at the center.

A 4' lengthening would be about 6' long with a crossmember at each end of the 4' addition. The new section was built before installation and added as a unit. The trucks were set on stands, measured, cut, cleaned up, and the new section was slid onto the existing frame, squared, leveled, clamped drilled and bolted, the the rear section was added to the new section and the process repeated. A new drive shaft section was made to fit between the Currier bearings.
They had a stack of frame sections made for orders, and the frame lengthening operation was completed in about a day, start to finish. The conversion of standard cabs to quad cabs was neat to see take place as well, but that took a little longer then a day. Gene