Personally, if I had a car with that many miles, I would run it stock until it dies. From the looks of that header, a 4-1 design does not seem like it would be a huge improvement over the stock manifold. As for fitment, you will most likely have to run a custom downpipe for the celica headers. From my experience with other headers, such as the Rippmods headers(a 4-2-1 design), the power increase is noticeable, but is by no means night and day. You may also run into a CEL like I have and a few other members by replacing the stock manifold.