The best replacement for 6.4 is 6.7.

The best alternatives to 6.4 are rated as following:
★★★★★  6.7
★★★★  6.0
★★★★  6.7 powerstroke
★★★★  7.3
★★★  6.7 ford
★★  6.6
★★  7.3 powerstroke
★★  5.9
★★  6.0 powerstroke
★★  lmm
★  lbz
★  5.7 hemi
★  6.1 hemi
★  6.7 diesel
  dodge 6.7
  6.0 diesel
  cummins 6.7
  ford 6.7

Your alternative for 6.4 is not listed? Suggest!
Speaking about 6.4, one probably shouldn't forget about dpf delete.

Other search terms related to 6.4 and their ratings are:
★★★★★ dpf delete
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6.4
6.7
6.0