Keith - 1st off, you've got a GREAT vintage trailer. You're gonna LOVE IT.
Your questions -
1 - the window should be rectangular, and yes it does matter.
2 - Go to Vintage Trailer Trailer Supply for the right part(s). Also read some of the threads here on the "how to". Not hard to do but in time consuming. You have to remove the window from the track, unscrew a bunch of hold down screws, put in the new glass you've had cut to fit, put all the parts 'n screws back in place as you removed them & slide the window back in its track above the window. You can do it.
I love that old time rock & roll.