I had the same issue with my 5.7 swap. It turned out to be the fuel injectors were gummed up from sitting. My 5.7 came out of a salvage yard and had sat for awhile before I tried to get it running. I pulled the injectors and placed them in a bowl filled with paint reducer and let it sit overnite. The next morning the reducer was brown in color. Reinstalled the injectors and fired right up