Hi from your spacers specification I assume you are fitting 5x100 PCD wheels to your GTO which is 5x114.3 and increasing the track by 50mm overall (25mm each side) the M12x1.5 is the OE stud thread so would be correct if you as using the same wheel nuts.
You do not mention the centre bore of the spacer/adapter you have but the hub of your car will be 67.1mm and the size of the centre bore of the spacer on the hub mounting side should be clearance 67.1mm
I have attached two photo’s of wheel spacers (not specifically for your application) the first is of a straight flat machined spacer which uses the studs to centre the wheels (Which I personally do not like) the second photo is of a Hubcentric spacer which gives a upstand in the centre, the diameter of which should be the same as the Centre Bore of the wheel you wish to mount IE: centre bore of your 5x100 wheel
I hope my explanation makes sense
