I recently fired up a rebuilt 440-6 on a test stand. Make sure the battery-starter connection is solid with a good ground. I had an issue with that. As others said, if you've done a lot of cranking, pull the intake and lube the cam. Check the intake runners, see if there is gas residue. May want to pull the timing chain cover and recheck cam/crank alignment. Be extra careful with the valley pan install. I had an intake leak that played havoc with startup and tuning. Re-verify CCW firing order.