really if your asking how this was done, you probably shouldn't be modifying hubs.
i say this because of the massive safety implications if you get it wrong
From the look of the part in the video I reckon that the new drive flanges were custom fabrications. The overall diameter looked to be exactly the same as the Celica ones though and if you were making them up from scratch you might increase the diameter marginally as the 4x108 stud pattern comes very close to the edge.
Maybe the originals were immersion welded to fill the existing stud holes and then the new holes drilled and splined? The rear drive flange looked to be a much larger diameter though and seemed to have a largish hole drilled through it (to access something?). I'm not familiar enough with ST185 parts to know if the standard rear drive flange is a larger diameter than the front one but seems a little odd.
Being able to convert to a different stud pattern might be of interest to some GT4 owners because of the limited supply of wheels available in 5x100 (with correct offset) nowadays. Easier if you are already running after market brakes and just need new bells, but not going to be a 5min job (or cheap!).
There are some Toyota parts available that allow conversion to 4x100 stud pattern but it's not a 'bolt-on' conversion.
-- Edited by davemk3 on Thursday 13th of August 2015 11:11:26 AM
really if your asking how this was done, you probably shouldn't be modifying hubs. i say this because of the massive safety implications if you get it wrong
@Bad Obsession: Thanks thought as much, better off not showing it.
@greyghost: I didn't ask how it was done (I already knew), I was just curious if the guys were going to show it but thanks for the condescending post, there's never enough of them on the Internet.