The first gear ratio of a 700r4 is a steep 3.06, the TH400 and 350 are right at is 2.52, depending on where you get your info. 4L80e are relative cheap to buy, and upgrade. Friend with a 700r4 in a 650 horse gen 1 427 cost him over 4000. I have ridden in that car and did not like the big jump to second gear, 4.11 rear gears.
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As mentioned above, 400, and 4l80e have 32 spline output, and as stated previously are fairly easy to install. However my 205 needed the large bearing in the TC, so it needed machining. You may need your driveshaft modified. The 4l80e needs a controller My favorite:
https://www.usshift.com/usq4rd.shtml. I say this so that you can be aware of some options I have tried/driven/had personal experience with. Bucket says that they are a direct . Study!!! I have a 700r4 that I could put behind my 6.2l Detroit diesel for cheap, however long term reliability for me is hands down the 4l80e. Mine has 45000 miles late model with all the upgrades 450 bucks. I will modify my drive line and tc myself, (journeyman machinist). bought the tail shaft adapter from Trans Dap couple hundred. All told less then 2000, but should last a lifetime. I will use the same trans behind the modified LQ4, or Modded 454, havn't decided which motor.