31 gallon blazer tank will be a tight fit on a SWB. If you got the smaller one (25 gallons?) it'll fit good. I used my stock crossmembers and welded some angle iron onto them to hold my burban tank. The biggest problem you'll run into is figuring out where to mount the filler.
Read this how-to, it pretty much sums it up and you can figure out the rest:
http://www.captkaoscustoms.com/blazertank.html