Just my $.02 worth, but I think you will find you must make some modification to your 3602 tank mount front bracket (if it is the correct bracket?) to make it work with your 3102 frame. The pictures below come from the Factory Assembly Manual (FAM) Kevin referred to above. Shown are the differences in the mounting designs and placement between the 3102 tank/brackets and the 3602 tank/brackets. More pictures of the frame diagrams show the differences in frame design between the 3100 and 3600 chassis's which needs to be considered in your using the 3602 brackets on a 3102 frame.
From the pictures in the FAM, the front and rear 3602 brackets look to be the same height at the frame mounting end, so not sure why yours won't fit inside the frame rails??? Pictures of your dilemma would help us understand better and may lead to better suggestions/help.
On a related note, the FAM shows the 3602 brackets work with a 3602 specific tank, different part number than the 3102 tank, so there may possibly be more to deal with using these brackets depending on what tank you have.
