Hi Bob!
For my 1958 boat - hull #37 - the seats are 10.5" wide and 5' long, with the thwart is 8" wide, unknown length, but filling the space between the side "tanks". I've long since cut up my original seats to recycle the wood into other projects, so I took the measurements off the scrap. Width is accurate, length notsomuch.
Photos:

- From Aft
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This is the overall layout. Note that my seat boards are also quite rotted, I propped them up with bags of leftover foam blocks to hold everything in place. The seat cushions are 14"x14", to give you a scale. Also note that the seat boards both curve to follow the line of the side-tanks. Lastly note that the seat boards are narrow enough to be completely outboard of the bilge re-enforcements (the dread hump running lengthwise on both sides). Just to give you a sense of the dimensions.

- Thwart
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This is the thwart setup. Note that there are angle--irons attached to the centerboard right below the thwart. The thwart is the same width as the angle-iron is long. Also this will give you your true length, from the transom bulkhead to the aft edge of the CenterBoard angle iron (line take out to the side-tanks, of course), about 60 inches.
As my boards were rotted, I completely redid them:

- New Seats
- 8b00dda1e7f71c138b00dda1e7f71c13IMG_20190417_121416.jpg (93.36 KiB) Viewed 9622 times
You'll note that my new seats are wider (16") and extend over the bilge re-enforcement "humps." My Thwart is also wider, 16" again. Still have that nice curve in the front of the seat, though! Of course, rule 5.5 of the Bylaws forbids one from changing the seat dimensions beyond the original seat. What can I say? My butt is bigger than 10.5 inches and <ahem> the Class Measurer wasn't around when I was rebuilding the seats. I throw myself at the mercy of the court!

Hope this helps,
Tom