What are you really saving? A halfway decent rotating assembly for a 306 starts at $895 (D&D Motorsports). With the machine work, you might save $500, but you would have a shortblock built to your specs, done to your quality standards. I have seen many engines from builders that were assembled incorrectly, causing way more headaches down the road. If you can't afford to do it right the first time, how are you going to pay for it the second?