Since you've already replaced the ECM and the DIS module (very expensive so far, I'd think), your problem may lie in either the wiring and connections between the engine and the ECM, between the engine and the DIS module, with the injector wiring or with the fuel supply. system.
Also, in the rare case where the SIR's DERM is disconnected, the engine won't start. If you find it discon. Reconnect it and try starting.
First, stop trying to run the engine on carb cleaner. Spray car cleaners are very low octane and may damage the engine.
Also, you said you have spark but didn't say how you tested for it. When you test spark, be careful how you do it. One wire at a time with all the other wires connected. If you do it other ways, you may damage the DIS module.
Next, you need the Factory Service Manual and you should start with Chart A3 "engine cranks but won't run" on page 6E3-A-21. If you don't have the FSM, you need to get one and '90 is the only year during the LT5's run where the book is two volumes. There is the big book then a "Section 8A Supplement" and you need both. CAC sponsor, Zip Products, usually has them in stock.
The "cranks but won't run" diagnostic is 4 pages and requires some time to do, but if you run that diagnostic you'll likely find the problem.
To run the cranks but won't run diagnostic, you need a scan tester, a 12v test light, a spark tester, an injector test light (or "noid light) and your fuel pressure gauge.
Run the first part on Chart A3 pg 1 and post the results.
