Changing oil pan

Hi, I have a Buick Century 1993, V6 engine, 3.3L. A mechanic has estimated that a constant oil leak was due to a rusty oil pan and that it had to be changed. I have tried removing all the visible screws of the oil pan. I have noticed howerver 2 other screws on the Rt side, that I could see way up, behind a narrow space that separates the oil pan from the car body. After almost an hour and with extreme difficulty I have managed to unscrew them going from the Rt wheel side. It seems that there is one more screw that I can neither see nor feel, again on the Rt side. I tried reaching it like the other 2 however the space is way too narrow to allow me to do so. Any suggestion will be much appreciated. I am sure there should be a civilized way of doing so. Thank you all in advance.