Looking through another Crash Book Illustrated which has the dimensions for the Chevelle and Pontiac frames, a comparison of the majority of the numbers (frame widths & lengths at differing points) are the same.
The Chevelle diagram seems more exacting using inches, ie 1/4", 3/8", 5/8" etc., while the Pontiac diagram uses decimals, ie .03, .64, .44, etc..
However, the dimensions still appear to be the same. The body mount at the cowl(front) to the body mount just past the axle are both exactly 110 1/4". The difference appears to be at the rear as geeteeohguy pointed out.
Now if my calculations are correct, there is a 3 inch difference between the Chevelle (shorter) and Pontiac rear frame rail from the body mount that is behind the rear axle to the rear most body mount nearest the bumper.
You may be able to "stretch" the rear of the rail by getting a piece of steel that is the same thickness and having it bent/formed by a fab shop. Or, you may be able to cut a section from your Pontiac rear rail and splice it in. If that is all that is needed, then really not a difficult job.
So it looks like it would work with some modification, but I would confirm the match by measuring by hand at a few key points.