Early 2011 15" MBP with broken discrete AMD GPU.

I have an early 2011 15" MBP with a broken AMD GPU.

I also have the 13" model which was successfully running OCLP. I thought I would see if I could resurrect the 15" machine by putting the SSD from the 13" in. It worked but there were some graphics artefacts.

I ran the post install driver update and after a reboot it just turns off.

I created a new installer flash drive and went through the install process, all of which worked fine, until the final reboot at which point after the loading bar moves about 30% the screen goes blank and nothing else happens. It seems like it's switching to the broken GPU and failing to boot.

Is there any way round this? Some way of forcing the discrete GPU to be disabled?

Thanks.