On second look, it seems like they missed a form:
"If the vehicle has been modified, such as the addition or removal of axles, or you are requesting a change to the manufacturer weight specifications, Form MV-426B must be completed."
Form MV-426B:
I am requesting a r street rod, r collectible or r modified vehicle. A detailed description of how the vehicle is configured (chassis, body components, engine, etc.) including all modifications, is attached. I understand my Pennsylvania Certificate of Title will be branded as a street rod, collectible or modified vehicle
This part of MV-426B confusing though:
Weight Certification - The weight certification for trucks, truck-tractors and trailers must be completed by a certified enhanced vehicle safety inspector if the correct weight(s) do not appear on the vehicle's certificate of title or salvage certificate (such as unladen weight, gross vehicle weight rating or gross combination weight rating). A weight slip for the unladen (empty) weight must be attached if the correct unladen weight does not appear on the certificate of title or salvage certificate. It is not necessary to complete the weight certification in order to change the registered gross weight of a vehicle. Previously titled vehicles that have been altered to raise or lower the gross vehicle weight rating (GVWR) must be certified by the vehicle manufacturer. A new VIN-specific weight certification decal must be issued for the vehicle by the manufacturer. This label must be affixed to the left (driver's side) door jamb or door latch post.
The part in green and red seem do disagree.