You might go for something like 3" X 4" rect. tubing... 1/8th. or 3/16th. ?? depending on the frame width. then blend the 7" existing frame into the 4" tubing... The boneyard might be a cheap source for a hitch for the rear.
There is a formula or ?? like you mentioned concerning the wheelbase vs the tongue to wheel length... In the attached thread (which is a Ford bed build) you can see the front part of my frame & drawbar... again it's extendable for flexibility I might want my hwy. towing length to be different from the trail length ?? I slid it into each frame rail about 16 or 18 in. then I tapered the truck frame to meet the new tongue tubing.
You might want to start with lining the trailer up to the Scout to see if the trailer is at the right height or needs to go up or down... then design the tongue deck & drawbar to meet your existing receiver on the Scout... if you're really wanting a shell then you might want the trailer lower due to the CoG of the set up, maybe have the new tongue come up to meet the Scout receiver... I actually swapped springs & hanger locations cause initially it was too high & too loose (as in the last pict. in the below thread) so I went to a slightly shorter springs to stiffen it up a little... I don't want too much sticking up above the bed rails... low CoG... Hopefully.
Maybe contact the guy who started the below thread see how he went about it...
http://www.expeditionportal.com/forum/threads/74179-Pick-up-Truck-Trailer-Exped-V-2
There is a formula or ?? like you mentioned concerning the wheelbase vs the tongue to wheel length... In the attached thread (which is a Ford bed build) you can see the front part of my frame & drawbar... again it's extendable for flexibility I might want my hwy. towing length to be different from the trail length ?? I slid it into each frame rail about 16 or 18 in. then I tapered the truck frame to meet the new tongue tubing.
You might want to start with lining the trailer up to the Scout to see if the trailer is at the right height or needs to go up or down... then design the tongue deck & drawbar to meet your existing receiver on the Scout... if you're really wanting a shell then you might want the trailer lower due to the CoG of the set up, maybe have the new tongue come up to meet the Scout receiver... I actually swapped springs & hanger locations cause initially it was too high & too loose (as in the last pict. in the below thread) so I went to a slightly shorter springs to stiffen it up a little... I don't want too much sticking up above the bed rails... low CoG... Hopefully.
Maybe contact the guy who started the below thread see how he went about it...
http://www.expeditionportal.com/forum/threads/74179-Pick-up-Truck-Trailer-Exped-V-2