Here's how I built my tandem axle converter dolly;
8.5 inch frame rails
2 chassis crossmembers
tandem suspension
2 axles
5th wheel
mudflap mounts
PMD tongue hitch
PMD pintle hook (on trailer)
frame rail tapered at rear;
front plate;
rear plate with tail light cutouts;
finished dolly;
under a trailer;
single and tandem axle dollies;
note 5th wheel kingpin placement of each