Gen 1 to Gen 2 pinion flange swap on gen 1 diff

Well 110 views and no answers. I’ll put up my findings here.

part number 19179934 is for a Gen 1 pinion flange kit and does not fit the larger Gen 2 driveshaft flange.

going to order the gen 2 flange. Unsure of correct seal for this flange but whatever. Will let you know if it fits!

Turns out the correct flange for 2009 Cadillac CTSV (according to GM Parts catalogue) is 25995850 which is:
Fits 4.251" OD cv joint or flange or "pilot"
30 spline
94mm bolt circle diameter

I will be mating to:
SONNAX T35-ALFY-07
1350 U joint
94mm bolt circle diameter
6 bolt
4.251" Flange OD

Driveshaft was made by Dynotech Engineering
Custom 1-piece aluminum 4" diameter 1350 U joints with billet flanges

2009 CTSV Pinion flange 25995850 fits into 2004 CTSV Getrag differential

GM Parts catalogue shows seal for 2004 and 2009 to be the exact same part number 25861283 but its WRONG! You will need to purchase the Gen 1 flange and seal kit for just the seal. It is not available seperately. I could not cross reference it with any other seal.

Measured outside diameter of sealing surface on old and new flanges and verified they are the same. But ID of differential snout is much largen on earlier models.

Sonnax flange T35-ALFY07 fits perfectly to 09 CTSV pinion flange 25995850 which fits perfectly onto gen 1 Getrag differentials. If you have an early style differential (gen 1 04-07) you will STILL NEED THE SEAL FROM 19179934

So recap:
DRIVESHAFT: Made by Dynotech Engineering
DIAMETER: 4"
FRONT FLANGE: Sonnax T35-GMFD-01K 3-Bolt
REAR FLANGE: Sonnax T35-ALFY07 6-bolt
JOINT: 1350

DIFFERENTIAL PINION FLANGE:
SPLINE: 30
GEN 1: GM 19179934 3.917" or 99.5mm CV style flange
GEN 2: GM 25995850 4.251" or 108mm CV style flange
SEALING SURFACE DIAMETER: 2.322" or 59mm

HARDWARE:
FRONT FLANGE:
3x M12x1.5x35mm Grade 12.9 Allen head
3x M12x1.5 grade 10.9+ nylock nuts or serrated flange nuts
REAR FLANGE:
6x M10x1.5x30mm

DIFFERENTIAL PINION FLANGE SEAL:
GM 19179934 pinion flange and seal kit - only available in kit, use seal only

IMPORTANT NOTE 2004-2007 PINION FLANGE/SEAL KIT PART #19179934 CONTAINS OLD STYLE PROBLEMATIC SEAL! GM HAS UPDATED SEAL DESIGN PART NUMBER 25861283 BUT ONLY WORKS WITH LATER MODEL DIFFERENTIAL SNOUTS.
GM Parts Notes: USE w/o CASTING# 6457038590 ON HOUSING DIFF DRV PINION GEAR, FOR 2ND DESIGN SEAL SERVICED SEPARATELY SEE 25861283; SPECIAL SUSP(FE4); INCLS FLANGE(86MM),SEAL & NUT; 1st Design

Fitting a 4 inch driveshaft was problematic on this vehicle due to tunnel size constraints. Heat shield on fuel tank had to be removed, but I suspect it has something to do with the aftermarket poly transmission mount bushing being used potentially sitting to high. Tried shimming transmission cross member with washers which helped, but don't love this solution. Driveshaft yolk boot is very close to bottom of shifter, had to shim shifter base up, but customer vehicle is using a home-brew Hurst shifter setup which required some modification to fit correctly. Will probably end up using some type of dynomat style heat shield to protect fuel tank, but room is extremely limited in tunnel.