Personally, I'd probably grab the EFI Connection timing cover setup and roll my own harness for an LSx PCM. This would allow me to take advantage of the OE parts availability/durability and access a huge network of LSx tuning knowledge.
...I assume the other options you're looking at still use the opti.
The modern aftermarket systems can run individual coil ignition like you'd find on a LSx or other current production vehicle. They just need a cam and crank signal, as would be outputted by the Optispark-delete timing cover setups from EFI Connection or Torqhead.
Really no reason to stick with the Optispark anymore unless its budgetary.
Why don't you have the stock wiring harness?
I'd wager it's because he switched to the Accel DFI system previously, and had no need to keep the stock engine harness.
Why can't you just pick up a stock wiring harness and use the torquehead kit?
Going forward...it's really the only reliable plug and play option.
If I were going through the effort of re-wiring the engine bay of an SS, I'd want to start with fresh wire that hasn't been exposed to 21+ years of heat and environmental exposure. At that point, why duplicate the stock harness?
He's likely beyond the point of 100% plug-and-play being a necessity. Either wire it with an aftermarket ECM harness, or fabricate one for an off-the-shelf LSx PCM. It makes no sense for him to saddle himself with the Torqhead-specific modified LSx hardware.
The Holley system does not come with injectors, so add another $350 to the tab.
None of the "universal" systems or those intended for retrofit on an OE multi-port injected engine come with injectors since they'd be dependent upon the engine's requirements. He should be able to re-use his current ones; no need to change them if they're sized appropriately.