Ok so I tried searching and my eyes hurt... lol

Installed 228r cam with stock casting 241 heads, the heads are milled .042" I am using .051" mls head gasket, LS7 lifters. My issue is as follows.

Ok so I was at a refutable shop here in SoCal, CMS, and Ryne, the owner, and I went out for a quick little street tune. My car had bad hesitation and would not idle well at all. Found out latr on that I had some plug wires loose. I had driven the car for about a week, roughly 50 miles with new ngk tr55 plugs left at the .055 gap they have out the box.

Well i checked everything got the wires plugged in correctly, now I changed spark plugs because we originally thought I had some badly fouled plugs, which now i see I kinda do have (they are black at plugs with only a weeks worth of use) I replaced spark plugs yesterday, took her for a test drive, she feels good at low rpms no real hesitation like before, but when i get on it on the freeway it has a slight hesitation, like a small misfire, was throwing the p0300 code which I know is common for h/c installs, I then went and replaced wires to see if maybe they were bad too, this morning took it for a test pass on the freeway before work and well still feels like she is misfiring a little on the top end. I floor it and it hesistates ever so slightly.

Also with the LS7 lifters and milled heads, 7.400" rods, could this cause a random missfire?

Any help is appreciated, she seems to only hesitate now on the top end, it is very slight but still a hesitation.