I would contact PBX and see how much they would charge for rails that fit your length requirements. For blocks, I saw some on eBay with both the top and side setscrew adjustments for 10 bucks shipped. Be sure to get the ones with the adjustments.

I ordered mine from glacern and am happy with my purchase but after seeing those blocks I think I may have been able to save a hundred or 2. Then again the cheaper rails seem to have different aluminum supports that are not as square and flat as the ones from glacern. Either way you go I'd get the 20mm rails.