first i'm going to assume this is a GM vortec 4.3L? I'm sure it is :o)

one of the first things i would verify is if you have the ground connected right.
there is a negative cable from the battery that connects to the frame/engine but there should also be a second ground strap aside from the battery cable to insure a good ground. Usually connected from the starter mount bolt to the frame.(it actually may not be on the starter mount bolt but it should be from the engine block to the frame) But generally if you turn the ignition on and the system powers on, dash lights up then your ground should be good and your problem is in the ignition circuit somewhere.

secondly, check the dash MIL light (it may be a specfic light on the display or it could be the neutral light flashing if it has one) see if it is flashing a code.
since you've already replaced the usual suspects check the main ignition fuse.

also check the oil pressure sensor, it too can cause a no spark scenario. (it's mounted right beside the distributor).
good luck
:o)