The GM Heritage Center archive did not have information on a "68 Suburban, but they did have an information kit on a '69 - which indicates a 3.07 axle base; 3.73 or 4.11 optional.
http://www.gmheritagecenter.com/gm-h...t-Suburban.pdf
(page 18, and page 33)
It did not specify between a TH350 or TH400, only that it was a TurboHydramatic trans.
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