Frank Miklos posted some photos on facebook titled "1958 Coronado Upholstery installation." I snitched a few of the motorbox rebuild because I'll be doing some similar work on the motorbox in my boat.

I had asked about the large openings in the side of the '58 box and Frank replied:
Grant: Century started putting those vents holes in by the 1st 1956. (maybe mid 1955) they were circles first through 1957. In 1958 they were made oval, then squared off in 1964 ish. I think century thought the big V-8s needed more air... The Resorter 16s did not get the vent till a later date...
I responded: Frank, thanks. I wondered because my friend's '73 'glas Resorter, nor my '81 'glas Resorter have the openings, and I can't imagine our engines not being able to get adequate combustion air via below the floorboards. I'd also think such openings would add to undesirable engine noise into the boat's cockpit. But I sure woudn't know. Thanks again.

Here are some of Frank's photos: