After several tests with the ARToolkit, I found it to be somewhat limited regarding the flexibility of changing the code. So I decided to try the ARToolKitPlus:, which is written in C++, it is very flexible in configuring my setup and it is still maintened.
Today I performed the calibration to the wide angle lens I am using, and managed to track markers up to a distance of 700 mm from the camera, in an area of approximately 1200 x 1200. It is a very loose setup, so further tests are required. However the results are very promising, as I believe this distance is being limited only by the camera resolution (640x480) which can be increased if I use other cameras.
This toolkit is already integrated in the LARToolkit!