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As I see it, what you want is possible and there are a couple of ways to achieve it. Both involve the use of xoops' centre-right block positions (top and bottom) to mimic the size and formatting of the right column and not placing/displaying blocks in the centre or bottom-centre positions.
I'm thinking that you want:
narrow left column|w-i-d-e c-e-n-t-r-e l-e-f-t c-o-l-u-m-n|narrow centre-right column|narrow left column
You can either:
1. use your theme's .css file to set the width of the centre-right blocks (top and bottom) to the same as that of the right column, leaving the centre-left (top and bottom) blocks as variable width (as they are in most standard variable width themes), or
2. rather less elegantly, edit your theme's theme.html file to hard-code the centre-right column width and remove the code that displays the centre and bottom-centre block (so that even if a block is placed there in blocks admin it won't display.
Does this help?