What parts are the same on swallowtails and other Mk1's?

I know that there are a few items on a swallowtail that are different than on a later mk1 (read panel, and I believe the bonnet is different?), dash only has 1 dial iirc? But how hard are swallowtail parts to find?

Like are interior bits different to others? 

Seen a swallowtail for sale but just wondering how hard it is to get swallowtail specific parts for it in the rebuild?