Hi, the bolts generally seize due to corrosion and oxidisation occurring between the dissimilar metals (steel and alloy). So, penetrating oil sometimes doesn't help too much as it cant get down between the threads (as they are bunged up with deposits)...but I still do use it, as it can help a bit. What I have found is it is a good idea to 'wake up' the bolt by giving it a good tap. I believe this helps fracture the deposits around the bolt threads. Always use a really good fitting Allen key, one that will not slip and round off, etc. Always clean the bolt head of 'stuff & dirt', especially where the key fits, as you want as good a fit as possible. Then, a gentle tap on the key to start the bolt moving as sometimes just applying a turning force can twist the head off. Once you get it going, do a small turn to undo, then counter that back to release tension in the bolt. Keep on doing this until the bolt comes out, do not think that as it is almost out you can just wind it out...it can bind and shear at any point (as I have found out ).

In other situations applying heat would generally help quite a bit, but in your case the components around would melt!

Good luck - oh the joys of these old cars!