I would stay at or above 150 - since fuel is not a great issue EFI 2.5 ( 2mpg) Mercs are reasonable and pretty bullet proof. If you go to 175 to 200 you can keep the RPM down and get to 2.5mpg as the weight doesn't change much. My 200 gives 27/28 cruise @ 3400, at 4000+ the fuel starts to go - the old a bigger motor works less comes into play. A local down here has one that he runs at 4200 and was complaining about 1.5mpg.
IF you can find a 2003 or later BRP Ram or Etec that would be a good choice, not sure what they weigh but my EFI comes in at 406. IMHO do not go below 130 if an etec is in the cards
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Any way you measure it - dumbass is expensive
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