MPG Motor Magnum vs. Hemi

Kind of a weird question and vehicle is definitely not an A-body, but this section seems to be the most knowledgeable out of all the different forums. I have a 62 Chrysler New Yorker Wagon with a tired 413 that in the near future would like to do an epic old school coast to coast family truckster type vacation. Say you had a $3000-4000 budget to do a newer injected motor swap what would you choose? It will definitely be injected and I already have a solid lockup 518 for the swap. Not looking for a 500 hp motor, but something I could possibly squeeze out the most miles per gallon in my big ole boat. I have a 2000 5.2 I’m my daily 63 Dodge Crewcab and love it. I look forward to get peoples opinions. Thanks!

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