I finally did it with a custome resolution for the camara at 4000x3000, it took 1h40m to render but it worked out ok, the PSD file is just 48MB, and I was able to change the resolution to 150DPI and get a final poster of 90inx67in, the customer is just happy, by the end of the day that's all what matter.
I will search for the "Genuine Fractal Plugin" just for future options.
Thanks a lot for your help,:thumbsup: