1983 Chevy Silverado will not start

Engine Mechanical problem
1983 Chevy Silverado V8 Two Wheel Drive Automatic

Two weeks ago I would have said that there is nothing on a chevy I could not fix, but this one has me stumped. My wife has an 83 chevy pick-up. Six months ago I put a 350 in it that came from a donor. AT the time I changed the distributor over to the HEI that came with the 83. It ran great for those six months but two weeks ago while my wife was at the lake it would not start. It didn't die, she just turned it off and it never started again. I pulled the plugs and checked to see if I had spark, and I did, but I changed the cap and rotor and coil anyway just to make sure. No good. I checked and made sure I was getting fuel, and I was. I thought maybe it had jumped time, so I put my timing light on it and the timing was still dead on. I am now out of ideas. The only time it acts like it might start is when you first go out to work on it, after it sits over night. Is it possible it could be in the carb, somehow not letting gas go through except what seeps through at night. I have tried starting fluid and that dos not work either. Any suggestions you have would be much appreciated.