On the WQHD mod:
Adapters from any of the eBay or aliexpress sellers should work. I've bought from eqstore on eBay.
For LCDs it's a lottery every time. I've bought from US seller browngranite, they sent me the wrong screen. I've bought from one Chinese seller, they sent the right one. I've bought from another Chinese seller, they sent the wrong one. The LCD I'm using now may not even be a true AUO B140QAN01.1 as its sticker says, Monitor Asset Manager IDs it as an LG Philips LP140QH1-SPB1, although it's possible the adapter is the one giving false readings by nature of its operation.
WQHD screens are quite expensive right now. It may be wise to wait a year or two for them to start popping up in the used market, at cheaper prices, in nearly perfect conditions. My experience with brand new screens is they all aggravatingly develop defects fast.
Because of the price and used-market-availability aspect the FHD mod is much easier and cheaper to perform right now, the benefits of WQHD notwithstanding.
On heatsinks:
I am disturbed by the reddit OP's modification of the T430's dGPU heatpipe but he says a hole in the pipe wasn't made. Copper is malleable, a dGPU heatpipe can probably be bent into a T420 without removing material. I agree that the connection made by the two pipes may improve heat transfer. Compare the T430 design to the T420 dGPU design, where the dGPU and CPU pipes are independent except at the vents, and therefore not helping.
In my observations of heatsinks in T410, T420, T430, X220, and X230, more heatpipes is better than less heatpipes everything else the same, and a copper interface between pipes and vents is better than an aluminum interface. For the T420/T430, the ideal heatsink with dual CPU heatpipes and a copper pipe-vent interface does not exist. T430 dGPU heatsink maybe gets close to accomplishing this.
Absent more dramatic modifications, modified 04W3269 (T430, dGPU, Toshiba fan+motor - motor to be replaced with T420's own motor) seems to be the best heatsink.
Edit 29 October 2019:
I've ordered a T430 dGPU heatsink.
There are three FRUs for T430 dGPU heatsinks:
04W3269 - Toshiba fan <-- I ordered this one
04W3270 - Delta fan
04X3788 - SWG Toshiba fan, looks same as other one??
From pictures what seems to make the T430 dGPU fan the best for quad core systems is because the main CPU pipe and dGPU pipe seem to touch each other for a large part of travel, which should result in faster heat transfer from CPU to exhaust vent fins. The reddit threat also claims the T430 heatsink has more fins. Regarding the T420 dGPU heatsinks, their iGPU and dGPU pipes don't touch and the dual CPU pipe versions offered are compromised with aluminum interfacing the pipes to the vents.
Furthermore the Toshiba fan versions are superior because the Toshiba fans fin faster and airflow seems to be directed better.
I theorize the CPU and dGPU heatpipes can be further bridged by bending the dGPU pipe to touch the CPU pipe more, or thermal-gluing a
small heatpipe over both of them.
Once my vanilla 04W3269 is in (ETA next week) I'll stress test my current fan, then install and test the 04W3269, before proceeding with any further mods.
Edit 31 October 2019:
04W3269 is installed.
Had to remove the original fan housing with the 5 pin fan and tape the original T420 fan housing with the 4 pin fan in its place. The rotor on the original fan is captive so that is kept. I just hammered the dGPU pipe instead of cutting into it. Additionally the left speaker had to be modified to fit over the dGPU pipe.
Thermal performance at maximum load is the same: bad, over 90C with sustained load, with throttling below max base clock.

Performance is possibly better at medium load.