Welcome to CHR! Look at the floor boards, the VERY back of the frame as well and the rear fenders. Under the bed too. The rear parts of these trucks and the back part of the front and rear fenders get the most rust because water sits in those area's after they had been driven. Just look all over the place though. Look at that chassis carefully though in the rear. If it's bad then a good donor chassis that is cheap if you find a right one at a junk yard would be a S10 frame. Good IFS front end and rear end will be great as well. Good luck on the project!