I know these are not the best angles, but they show how my truck sits with appx 3" lowering blocks, stock s10 bed bottom sitting on its stock rails with respect to the frame (I grafted the '54 bed sides to the S10 metal bed bottom).
Rear Wheels are 17" and I believe the rear tires are 255 50's. You are correct that the smaller wheel will bring the truck closer to the ground, but the delta off the ground is only half as much as the total difference in diameter. Both the lowering blocks and the spring rearching will allow you to drop the CG.
