Honestly, if you want hi-po parts for the thing (which there is an overabundance of) I woudln't go thru JCWhitney.... check out Jegs or Summit or something, I would think they would have much better brands.
If you are messing with the torque converter, then I would guess there is already a shift kit in it? Cadillac got the softest shifting calibrations... wears down the clutches more, tho the TH400 really dosn't care... but you can get that thing slamming into gear faster then any manual... of course depending on your goal, you may not want that firm of a shift... all depends on the setup... but stock, even under WOT, hardly feel the shifts.
Automobile(s): 1995 ETC, 75 Deville, Cad500 powered 73 Apollo, 94 Mark VIII
Re: 425 transmission
You know, it's interesting that you brought this up.
On a modern automatic car one of the first modifications is a high stall convertor. Not as much attention is paid to the convertor in older cars, almost none in Caddys. Then again a modern auto will have a lockup convertor and a TH400 won't, so highway economy will suffer. Either way there's no denying the performance benefit, a guy here in Albuquerque went from a 16.5 best to a 15.78 changing to a 3000 stall in his Mark VIII.
Keep in mind having a really high torque motor will cause the convertor to stall higher than the advertised RPM.