blazer ???
 

Does the blazer rear seat sit exactly over the rear axel in between the tubs?

basically, thats where the underfloor flat brace deal is that is for the mounting of the seat, i remember that much form when i tore up the 72 blazer tub

There are metal brackets in the floor where it bolts down. If you have one you could look at, that would show you where the seat sits. Someone had photos from cleaning up the bed that shows the brackets. I believe it is right between the wheel tubs.

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'69 K-5 Blazer - 350/TH350, 6" lift, 35" tires
Bothell, WA

On the underside of the bed, there should be some encased nuts for the seat brackets. At least mine did, and yes, it sits right between the wheel-wells.