Go to a title and tag place and get an application, its a few pages. I think you are getting what is referred to as a reconstituted title. You'll need to have pictures from each angle, inspected by a certified trailer guy to have a Gross Weight applied, you'll have to list all the materials and costs in the construction, possibly with receipts for the materials. It is a major pain and can be pretty expensive.
http://www.dmv.state.pa.us/pdotforms/fact_sheets/Specially_Constructed_Vehicle.pdf
http://www.dmv.state.pa.us/pdotforms/mv_forms/mv-426B.pdf
http://www.dmv.state.pa.us/pdotforms/fact_sheets/Specially_Constructed_Vehicle.pdf
http://www.dmv.state.pa.us/pdotforms/mv_forms/mv-426B.pdf