Eye sighting may be very hard to do, and see, in the 1/8-1/4 range. There may be humps, rivets, metal overlap and all manor of features to interrupt sight, unlike a board. Getting your eye down there (with a level head
) is harder than standing and holding a board.
This point has been made by other posters already but here are my thoughts. 1/8 is in the area of level, on a truck that old. You may be thinking there is a "problem" that does not exists. The frame build tolerances, together with metal and rivet fatigue/stretch, would give 1/8 across 34" easily. How are you arriving at the 1/8-1/4 dimension? How are you measuring that? There could be some error with how you do it.
By the time you add the thickness tolerance stack, the fatigue and shrinkage of the hardware/pads that make up cab mounting, you could even be 1/8-1/4 in the opposite direction, regarding your results in checking cab for level. Your numbers, 1/8-1/4 are not abnormal even if you are exact. So my answer is no, you do not have a problem to worry about.
I have a laser level on a tripod that puts out a horizontal and vertical red line after the tripod is bubble leveled. I use that for all kinds of things around the house. This would be a good thing to use in your case if you want to be sure. The sagging springs is a good point. That has to be checked and/or taken out of the equation. (this is the point Mark made about putting frame on leveled jack stands)
A 20% coupon available in many fliers and magazines, makes this $44.https://www.harborfreight.com/self-leveling-laser-level-kit-69243.html