Nice work! Have you got any pics? Mk1's are indeed rare now. Do you have a pic of the instrument cluster? I can't think what a 1978 dash looks like.... I assume it's the same as an up to 82 model year before the later Mk2-Mk3 dashboard.
Is it like this?
You might want to check out volvo300mania.com as someone on there might be able to help as well