Here's a pic of the radio in my 1968 Newport.
The 2 vents below the radio are only for A/C equipped cars. (if memory serves correct) They can be unscrewed and non-A/C block off plates installed if needed.
The overlay trim that you're missing is glued on. It's silver for Newport and woodgrain on...
Here are a few reviews for SMS, read them all.
tl;dr - one of few sources for rare upholstery, pay upfront, wait 6 months to two years for delivery, horrible customer service.
Your new upholstery looks nice! To remove the rear seat - the base has the typical loop/hook so push the base inward (towards the trunk) then lift up. the back has 2 bolts at the bottom then simply lifts up and out.