How do I measure replacement window - center front window?

Pull the eyebrow, Pull the four small windows out by sliding them out the side, Remove the entire front window assembly frame, Lay it flat on your bench, Clean all of the old butyl glass seal (or PO silicone) out of the frame, Take the frame to "a real glass shop" and have them measure and cut the replacement glass. If the glass is "to large" by only a small fraction, it will NOT fit, and you cannot remove "just a little" extra glass. My local glass shop would charge $17 to $20 for the glass cut to your required size.

I would NOT have, or pay the glass shop install the glass. You will do a much better job, installing it yourself. The glass shop will likely just glue the new glass into the frame, and it will be a mess if you ever need to repair a cracked glass, in the trailer. Make sure you have the lips around the edge very clean to allow the retaining clips to secure the glass after you put the butyl tape on the frame and spray it with Windex so it will not stick while your installing it. Then lay it flat out in the sun for a day to dry. It's easy to replace the glass with the frame still in the trailer, the hard part is cleaning everything, and getting the retaining strips out of a window that was smeared with silicone, which in my opinion is almost all of these old Hehr Standard windows.

You will find that you should "restore" the entire window in the proper step order. It would be silly to go to all the effort to fix the center glass, and not replace the screens and polish the rest of the frame while it's apart. I have spent up to 40 hours restoring a small five Paine front window, you also need about a $100 bucks in parts, hardware, and various glass and backframe seals. Don't even try to replace the screens with anything but Galvanized Steel, Not Fiberglass, or Aluminum screen. You can buy the special "steel" screen at VTS, or McMaster Carr.

I have seen that video only a few days ago which was after the initial posting of this thread.  

I later discovered through Larry's video, that the black stuff was the squashed putty tape and that some of it had, I think squeezed out about 1/8" on both sides of the glass and hardened.  It had dried over time and did not come out with the other black stuff when I cleaned out the frame with a putty knife.  I thought it was initially part of the construction of the frame. So, I thought it was a "stop" on the sides.  I just wanted to know the clearance allowance on the sides, top and bottom.  I just assumed having a fudge factor of a 1/8 on each side would be acceptable and then center it.

BTW, John, thanks for your time in describing the install.

Cheers

Rod