not sure what you mean when you say the idle is getting worse. What is it doing? Idle too low? Idle rough? Stall?
If you could hook it up to a scanner you could look at your fuel trims to see if it's going rich, lean or okay. you could look at your idle air control to see if it's within a normal range and acting smoothly. How is the spray pattern out of the injectors? Is the fuel pressure within specs? (something to look at even if it is rebuilt)
How's the quality of the fuel in your area? Sometimes the ethanol content isn't what they say it is. You might want to try a fuel system cleaner. Keep that new engine clean on the inside.
you could hook it up on a 5 gas or even a 4 gas engine analyzer and see how the exhaust is coming out of the tailpipe. If it's coming out clean (less than 220 hydrocarbons and less that 1.0 CO at idle the engine is running good. (according to state emissions...)
what do you think the distributor is dong? bad pick up coil? Module? is the engine missing? If it is can you tell which cylinder or cylinders? An engine scope could look at the secondary ignition and see how each of the cylinders are firing. It could pick up on a spark plug
or wire that is not firing correctly. If a cylinder is running out of fuel too soon compared to the other cylinders. Or too rich. Too high KV could show up if resistance is too high for the spark to jump. Most scopes have a test for running cylinder balance. It tests the running engine to see it they are all firing evenly. Check to see if you have any cylinders that have a lower contribution that the others.
you said that you replaced the distributor. Did you have a problem with the old one? what was that one doing?
let me know what happening and tell me if you can describe the concern you have about the distributor. And how the idle is getting worse.