I have installed a replacement tongue on a 40 x 48 to lengthen it for better towing and simply bolted the 9' +/- (???)
2x2x3/16 at every cross member With a single grade #5 5/16 bolt at each member. It now has a opening that can be shimmed down to accommodate a class #2 (1 1/14) bike rack or extension platform. I learned a trick, use a square bottom "u" clamp at the last connection to keep the tube clear for insertion.
As to the 4x8 HF trailer I added 18 inches in length to the forward length and inserted the 2 x 2 x 3/16 square tube as a reinforcement to the rather anemic "A" point tongue. Again simply bolting at every cross member. Although it runs over the axle there is no indication it has ever bottomed out.
We sort of discussed this earlier this week here is the thread with pics.
http://tnttt.com/viewtopic.php?f=35&t=60152&p=1083792#p1083792