Chevy Truck no spark

I am restoring my 77 chevy 3/4 ton pickup. I replaced the engine last fall and tried to start it. I have not rebuilt the carb then so I got it to pop off and run for a few seconds with starting fluid. This spring, I tried to start it with everything hooked up and it won't gererate a spark. I have replaced the distributer cap and rotor, spark plugs and wires and I think the coil. Can't remember. It's an eletronic ignition and was wondering what could be causing the problem of no spark. I took out a plug and grounded it to the frame to find out that that was the problem, so timing wouldn't matter. I don't want to just start buying parts until I hit the right one.