I had a similar problem. Changed just about everything I could without taking the engine to bits and it ran just about OK, but not more. Not enough power, didn't respond to timing changes very well. Started each and every time, though, but ran much too rich. Sounds similar to your poblem. even the spark plugs were OK, only had a bit too much carbon on because of the rich mixture.
I did a compression test and found out what was wrong: The head gasket between #3 and #5 had blown, causing low comp on those cylinders and a poor burn rate. In addition, the passenger side head had a small crack and #4 cylinder was half full of water. You can fiddle with everything on top of the engine and it might get a little better or a little worse, but SBCs can run with quite a bit wrong down inside them. My guess is do a comp test, it's easy to do and if you find cylinder pressures differing by more then 10 or 15 psi, pull the heads off and have a look. Might cost you a day and some new head gaskets, but if the engine doesn't respond to outer accessories being changed it's probably got something to do with the innards. Especially if it's your first build it isn't even all that easy to get the head gaskets to seal properly. If you didn't tap the cylinder head bolt threads properly or some of the bolts were dirty or had rust or scale on them then your torque readings go all over the place and the cylinder heads are the #1 place to get the torque right exactly as specced. Of course it could be something simpler than that, but if it is a head gasket, the time to change it is now. Hope I could help a little, MadMax