Bolt a 700R4 or sbc 4L60E with the 700r4 or earlier 4L60E converter to a Gen3 or 4 sbc engine without a spacer, shims, diff flexplate or any form of adaptor kit or adaptation. If it works, runs and drives, Ill stand corrected.Bolts right to a 4.8L or 6.0L that came with a 4L80E. As I said earlier my 6.0L bolted right to a TH400.
No one other than you is talking about fitting a TH400 nor are we even talking about TH400 interchangeability/compatobility across those engines. No one has denied a 4L80E will work on all generations of small block (gen1-4) as is.