Interesting that you should bring up the issue of Merlin engines on those Canadian C-54s vs. the radials used by the Yanks. Your man Don McVicar had an opinion on this very subject. He thought the Canadian decision to go with Merlins was a dumb idea for two reasons. 1) the in-lines were much noisier than the radials, and 2) they had a much shorter service life. They were great for war planes, where service life was basically no big deal, but for a civil airliner, which would operate the engines for longer periods of time and more often, the choice of the in-lines was a bad business decision.
Don had an opinion on most everything
. To a degree he was right.
The inlines had a higher mtce cost in part because they lacked the abundance of cheap parts and had a numerical disadvantage in demobilized engine techs as well. It was easy to order up a jug for a R-2000 and slap it on (which could be done in the bush if required-- and often was) but the inlines required an engine change if there was a serious failure. One reason the Canadair project was spec'd with the Merlin was it's efficiencies while both Canada and Britain HAD the Merlin techs. On the downside, the inlines had the additional complexity of a liquid coolant system.
As far as longevity, the numbers are skewed somewhat as there were few transport a/c with inlines and the combat a/c certainly did get pushed to the limit regularly. Even so, the most fuel-efficient fighter of WWII with one of the lowest mtce-hour/flight hour was the Mustang.
That the NorthStar/Argonaut was operated successfully around the world well into the '60s (and often in 'remote' locations) does indicate that in airline service they were at least competitive, but couldn't compare overall because of the small production numbers.
The telling argument is that due to the source of most cheap, available a/c being US-made transports ( hundreds of DC-3's & DC-4's for example) it was difficult for ANY new production to gain a foothold. By then Douglas had a leg up on the next-generation. P&W and to a smaller degree, Wright had swamped the world with radials with a huge inventory of parts while people like Rolls were limited by post-war recovery.
It would be interesting to see what a level playing field would have brought in the way of performance, fuel efficiency, simplified cooling
and other improvements. As a side note on the noise issue, TCA developed the MacLeod crossover exhaust system for the Merlin which (at least for the passengers) reduced the noise significantly in much the same way that the collector-ring exhaust of the P&W's did - by moving most of the exhaust outboard. The short, direct stacks on the Merlins aimed right at the passenger cabin WERE brutal!
Willy, I suspected the 307 would do well on fuel numbers. Where it would be questionable in an airline environment would be the speed and load. Boeing tried the quantum leap to the Stratocruiser, but cost and time were the gotchas... the corncob WAS a gas-hog and of course, by the time it was 'civilianized' the jets were coming. It was too big a leap with too little time to prove itself while Douglas could enjoy huge production volumes with a maturing DC-4, 6, 7 production run.
Looking back at the table I see a potential comparison to the auto industry that we best discuss elsewhere
Ultimately, dang jets ruined a wonderful struggle
Some interesting data if you compare these two pages side-by-side:
http://en.wikipedia.org/wiki/Pratt_&_Whitney_R-2800
http://en.wikipedia.org/wiki/Rolls-Royce_Merlin
Rob
and one from John 
and one from Rob