[Mark Hewitt]

Since it's the first post it would be better to associate it with a thread, rather than put extra fields into each post.

You probably know all this stuff already but.
Alter the thread table so every entry has new fields to store the required information.
You need to change the template for new thread to include your new input boxes.
Then find the part in new thread where it accesses the database thread table to create a new thread. You need to alter this to include your new variables that are passed to it by your template modifications. Looking at the code you can probably tell the syntax.

Once the information is in the database you need to find a way to get it out. Using postbit probably isn't the best idea as that deals with every post not just the first one. Perhaps showing the information at the top of each thread would be acceptable?

You might be able to get away with not altering showthread.php itself as IIRC the entire thread record is downloaded so you should be able to put it directly in your templates as a variable.

Sorry if it doesn't tell you exactly how to do it, but to do that I would have to implement it, and I don't need this feature!