I think it also comes down to warranty... If Quigley can do a conversion using mainly Ford parts and sell them on Ford lots, then warranty work is:
1) easier for the Ford tech to do since the parts are stocked in their warehouses
2) familiar parts for the Ford techs to use
3) the parts are not made by Quigley, so if a coil spring sags or breaks due to manufacturer defect, then it's on Ford, not Quigley
IMHO it's a smart way to build. It ensures that even if Quigley goes under, that the van owners can still find parts and service.