If you need to modify your DNA construct, one frequently used method is the PCR-based mutagenesis, where you incorporate the mutation into the middle of two complementary primers.
My last NEBuilder assembly got stuck because I couldn't get two of the PCR reactions to work. So I tested alternatives to the Phusion DNA polymerase, which I have been using exclusively for cloning purposes during the last years...
How much PCR product does one get from a typical PCR reaction? 50-120 ng/µl seems to be a typical result, but it very much depends on your PCR conditions.