With 4000hr in Twin Otters ill say this (yes its a long tine going nowhere)
Standard Fuel capacity is a little over 2400lb, PT-6's (-27/-34) plan on 600lb/hr all day every day doing utility work low level
at regular MTOW 12,500lb and an high (dont laugh) cruise FL100+ you can get fuel flows down to the 520lb/hr mark.
With a ferry permit the highest weight i've seen was 17500lb, it used a lot of runway and a lot higher than usual take off speed. If you lost one on departure then close the other throttle and land ...... wherever that is. Our ferry kit in the day was a wood base with the cut out for a bunch of 44 gal drums connected by 2 inch hose that fed a manifold into the two under floor tanks.
Any ferry was preceded by a flight test to ensure the ferry set up would work and there were no blockages. Usually we would climb on the mains for 90 min after departure then open the feed on the ferry tanks and see if the fuel gauges increased, if not we'd turn back.
My opinion in this case is for whatever reason the ferry set up would not feed the main tanks, The crew exhausted there mains and were screwed after that.