The best replacement for th400 is th350.

The best alternatives to th400 are rated as following:
★★★★★  th350
★★★★★  700r4
★★★★  powerglide
★★★  4l80e
★★  4l60e
★★  c6
★★  4l80
★★  c4
★  t56
★  th700
★  sm465
★  tf727
★  th350 transmission
★  700r4 length
★  727
  200r4
  v160
  th700r4
  700 r4

Your alternative for th400 is not listed? Suggest!
Speaking about th400, one probably shouldn't forget about c5 diff.

Other search terms related to th400 and their ratings are:
★★★★★ c5 diff
★ 4l80e
You have in mind a companion for th400 and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of th400 in all years. Click on graph for more.
th400
th350
700r4