My bus had windows that were about 66 X 30", except for the upper front and rear corners which were about 10" shorter with a slightly slanted side edge (right along the corner).
All the windows you can see in this photo are Peninsula except for the central upstairs one (the one that's a little lighter in color in the photo -- that's the one in the stairwell - it doesn't open). One of my issues was that the stock windows had 4" radius corners and the only Peninsula corners were 3". To make them fit, I had to trim out a crescent in the sheet aluminum to make the new windows fit. I measured to a pretty tight size for the upper and side straight edges and the final parts were very accurate to the size.
As I got in the middle of the project, I realized that as long as i was trimming the corners, I could have as easily ordered the corners square -- that might have given me a more modern look and might have been easier to fit. So, don't forget to "think out of the box" as you're considering your options.
I've found that if I crack the windows downstairs on the side that the breeze if blowing from and open windows upstairs on the downwind side (all the Peninsula windows have screens), I get a cool breeze up the stairs and the air in the bus changes quickly. In the past few weeks, I've been parked in the sun with temps close to 90 and NC humidity and the bus has been reasonably cool inside.