The Electra's usual fuel tanks aren't big enough, especially for the longest leg of about 14.5 hours from Hawaii to California, so the fuselage will be fitted with a "giant bladder" to carry extra aviation gas to spin the two radial piston engines.
Mackley loves his plane.
"It's a beautiful looking thing," he says, "art deco through and through. It's like a mirror, a thing of huge beauty. It was the first twin built by Lockheed."
It was also the American company's first metal-skin plane and, in its more than 20-year restoration project, Mackley's Electra has had many of the panels replaced. Its Pratt and Whitney Wasp Junior engines, too, are virtually new, following international searches for crankshafts and many other spare parts and a rebuild in Palmerston North.
Mackley's late father Bill was an Electra pilot for the National Airways Corporation and earlier, as a teenager, saw the mangled, burnt remains of New Zealand's first fatal Electra crash at Mangere, Auckland in 1938. Rob has a piece of metal with a Lockheed logo from one of the tails of that plane.
His Electra began its working life in 1941 for a Chilean airline. In 1959 it was sold and later went to the Alaska.
In the mid-60s it was impounded by an Alaskan state authority over unpaid bills and given to a transport museum. In the 1980s it was offered for use in an Amelia Earhart reinactment project, but the deal never came off. It later went to another Alaskan museum and, to help offset losses on an airshow, it was put up for tender. Mackley won, with an undisclosed price.
It is one of 14 complete and two incomplete twin-engined Electras remaining worldwide. One whole plane, and parts of another, are at the Motat museum in Auckland. It is understood only two - in Canada and the Czech Republic - are airworthy. Mackley's will be a third.
He previously owned a DC-3, but considered it too big for one person. He couldn't afford to buy a large enough hangar and having to leave it outside in the weather meant it would deteriorate too fast. The Electra has a shorter wingspan.
Asked about major hurdles in the restoration, Mackley said there was none, "other than financial - it's just a big hole for money".