Rebuilt short blocks

Has anybody used a parts store rebuilt short block instead of rebuilding your own. I have collected parts to build a Magnum head 318 or 360 to use while I am rebuilding the original HP273 in my 66 Dart. I found a couple of brand name rebuilds for $500 + freight. It's pretty hard to beat that price at most machine shops. I allready have a cheap used 318 in the garage but it's only core material.(I was hoping to pick up a decent 318 and just bolt on some parts and go.) Thanks Mike