Take a look at this write up:
http://www.coloradok5.com/full-conve...nversion.shtml
That should get you most of the way there. Keep this in mind when chopping off the half cab of a later blazer, support. The factory full convertibles (73-75) had rocker boxes under the tub to make up for the support lost by not having a steel roof. You should add support back in if you plan on running without the top in the summer. Otherwise, the tub might sag with the roof off making the doors hard to open or close.
the 4x4 guys just end up with a cage to add the support back in, if yours is 2wd a full on roll cage might be out of place. You will have to do something to simulate what the stock rocker boxes do under the tub.