[SOLVED] Different widths for pages and posts

I thought I could easily figure this out but am finally losing my mind... so Help, please!

The page width for the site I am building is 1600px, which I have set in the ATO / Layout. The posts, on both the single posts and the homepage for posts needs a width of 1000px. When I try using using left/right margins, e.g.

div.post {margin-right: 300px;}

but the right sidebar (used only on posts, not used on pages) stays far to the right. I need to bring both left and right sides in 300px each.

I've tried setting the layout width to 1000px and changing each page's width in CSS to 1600px but having been unsuccessful using variations of:

.page-id-51 #content {
margin-left: -300px;
}

or

body.page-id-11 .container {
max-width: 1600px;

but have had no success. I only have 5 pages so it's not a problem to style them individually, if I could only get it to work.

What am I missing that should be obvious? Thank you or your help.