Thanks for the advice guys. Do you know what size spacers are required and would there be a need to fit longer studs.
With an original GTV wheel removed from the car, push an original GTV wheel bolt through the wheel, and measure how much protrudes. Now, do the same with your replacement 156 wheel. You will notice, that much more of the GTV bolt protrudes through the 156 wheel, so much so that you will not be able to properly tighten the wheel bolts; they will try and cut their own thread into the hub face.
If you are using 156 wheels, you should use 156 wheel bolts. However, if you are using 156 wheels and spacers, you need custom wheel bolts, the length of 156 wheel bolts plus whatever thickness of spacer you are using. Or you can cut down your original GTV wheel bolts to make them the correct length.
In light of what youíve said regarding inadequate coverage of the spigot am I right to assume the 15mm hubcentric spacers would suffer the same problem?
Were you able to fit 5mm spacers without the need to fit longer studs?
Would you happen to have a photo of your car showing how the wheels look please?
15mm hubcentric spacers have enough material to keep the strength in their built in spigot. 10mm hubcentric spacers definitely do not, and 12mm hubcentric spacers I reckon are borderline, I wouldn't use them myself.
If you use 5mm spacers then you can use your GTV wheel bolts but I think you will need to shorten them by a few mm each. If you use 15mm hubcentric spacers then even GTV wheel bolts will be too short, custom bolts will be your only option.
Hope this helps.