Wrong...you need an outer ring that's not a "trials" ring. Get one that's 32 or 34t.

40G, 63G, 90G: Weight in grams

You will need to remove the crankset to do the inner. It won't fit around the spider.

Regarding your spacing...this is why I suggested you move one of the 2.5mm bottom bracket spacers from the right to the left side. (assuming it's outboard) It would definitely work for ya...I've just never understood why someone would do something like that. It's not any more "romantically simple" to have a 5spd cassette than it is 9 or 10. You still have the same amount of control / derailleur / cable clutter. If you want simple...bag this idea and go singlespeed. You would be surprised at how much stronger you will be in just 2wks.