Welcome to the board.
As far as posting you just need to make sure that your post is in the appropriate section for instance this is the projects and builds section so this is the wrong section for this post but seeing how you are new here we will let it slide
Yes Highlanders are relatively rare however since 67-72 C10's aren't considered a "collectable" they aren't going to bring huge money.
The short bed trucks are bringing the money right now whether you are talking about a purist restoration or a fully modified truck slammed on the ground with bags.
The long bed trucks aren't recieving as much attention in the Mod crowd but there are a few here and there, mostly guys just go back to stock with them.
A stock restored base longbed depending on the level of restoration will bring 8 -12k and for a Highlander 10-14k, a 4 wheel drive can fetch up to a couple thousand more.
As far as how many were produced I haven't looked it up.