Don't have my books with me, but in general, the 3100 frame is tapered all the way back, and the 3600 frame matches the 3100 from the front to somewhere under the box, then the rails are parallel. Wheel openings are the same, as they use the same fenders. Wheel base is (I think) 9 inches longer on the 3600.

Hope this helps.

Current fleet (subject to change w/o notice)
\'49 GMC 3/4-Ton , 60 Austin Healey Frog-eye Sprite (some assembly required), 2011 Dodge Avenger, 2015 Jeep Cherokee. No, they don't all run.
My other ride is a (B737)no, (B767)no, A320.... Update... was Embraer E190, now Boeing B787.

Knowledge is a wonderful thing, but ignorance means you don't know you can't do something.