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The O2 being down low by the Y-pipe is likely due to the heavier GVWR.
3L80E (TH400) is simply because there was no heavy duty 4 speed overdrive automatic in 1989; 4L80E didn't hit the scene until 1991.
That bare-bones interior trim is somewhat common on the Cheyenne trucks; I've had several of them like that as parts trucks. They don't even have B-pillar trim, just a little grille over the speaker.
No dash vents - now that is just plain crazy! Hadn't seen that one before.
Tow hooks are an extra cost option on many models - I don't see it on your SPID label; guessing someone added them?
I noticed...no hood insulation pad.
Richard
I’ve only seen the no dash vents on here, never in person