The best replacement for 7.3 is 6.0.

The best alternatives to 7.3 are rated as following:
★★★★★  6.0
★★★★  5.9
★★★★  6.0 powerstroke
★★★  6.6
★★★  6.7
★★  cummins
★★  6.4
★★  6.0 diesel
★  6.4 powerstroke
★  v10
★  6.0 excursion
★  12v
★  6.0 mpg
★  lb7
★  duramax
★  6.5
★  12v cummins
★  12 valve
★  5.9 cummins
  5.4
  6.0 ford diesel
  6.9
  6.0 ford
  460
  powerstroke
  6.0 liter
  6.0 powerstroke diesel
  6.8
  7.2
  6.7 powerstroke
  chevy 6.6
  dodge 5.9
  6.5 diesel

Your alternative for 7.3 is not listed? Suggest!
Speaking about 7.3, one probably shouldn't forget about edge.

Other search terms related to 7.3 and their ratings are:
★★★★★ edge
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7.3
6.0
5.9