Working on this for 3 years, Can you HELP me, Please?

I have a 2007 Trailblazer SS, 68276 miles, VIN 1GNES13H272304086,   364ci  LS2, automatic. Removed engine and transmission 3 years ago to do upgrades. Power cam, springs, retainers, pushrods, oil pump, timing chain, changed oil pick up location, headers, intake manifold. Trans: torque converter to 3200 stall, upgrade sun shield, other Sonax upgrade parts. Put engine/trans back in, started right up. Did not have first or forth gear. Took everything back out and went through trans again. Reinstall. Had communication with ECM but NO crank, NO start. Used remote switch, motor turns over freely. Then my code reader says no communication with ECM. Has power to Data Port. I watched one of your videos that said quick way to check communication is by check engine light. Turn key to run position. Engine light comes on for 12 seconds and goes out. What should I look for now??? Battery fully charged by Battery Tender. Tried 3 different ignition switches. Took junction/fuse block out 3 times. I'm 74 and I'd like to go for a ride while I can still drive.  Please advise, Thank you