1998 Chevy Silverado Rebuilt engine multiple or random misf

1998 Chevy k1500 350 129000 miles

I recently rebuilt this engine and did all the mechanical work myself. I had a very experienced guy do the block work, and he put a new crank in it. I kept the same cam and heads. The mechanic that did the block said the cam was alright, and the heads/valves appeared alright to myself and several others. After several hassles and tinkering I am to the point where the engine will run but brings up the po300 trouble code, twice. I have been through the ignition system several times, and everything appears fine. I have no compression tester, and did the valves according to a haynes manual. What next? I am getting limited on what I can do myself and dont wanna go broke at the shop. Any suggestions?