2001 GMC Jimmy Engine will not turnover

A 2001 GMC Jimmy Vin: 1GKCT18W01K150784, Model: SLS 4X4 Engine: 4.3L Vortex (V6/262 cubic inch) CSFI (Central Sequential Fuel Injection). I changed the fuel pressure regulatory on the CSFI. After completing the removal and replacement of the fuel pressure regulatory. I reassembly the air plenum components etc, the engine initial turnover without starting then backfire. A couple more tries at starting the vehicle with no success

After that the starter would engage the flywheel and all you would hear was a clunk the starter was engage but could not rotate the engine. I followed all the normal troubleshooting instructions. Check the battery, the battery cables, no problem noted but I did change out the positive battery cable from the starter to the battery. I removed and replaced the start same results a clunk from the starter engaging the flywheel.

I decided to retrace my steps and removed the air plenum and check the installation of the fuel pressure regulatory. I found when you would turn the key on the fuel pump would pressurize and fuel sprayed out of the fuel pressure regulatory into the manifold. Once I inspected the manifold I noted that the air passage were full of gas, to the combustion chamber. I reseated the fuel pressure regulatory and insured I had the retaining clamp installed correctly. I turn the key on and no gas leaks, problem solved for pressure regulatory.

I pumped the fuel out of the manifold passage way approximately 16 oz. Of gas removed. Then I inspected the manifold passage way with an inspection camera and could see fuel further down in the manifold passage leading me to believe that the combustion chamber was flooded with gas. Pulled 4 of the 6 plugs and only found the #1 plug wet with fuel residue. The rest were dry.

I then pulled the oil dip stick and found the stick indicator showed no evidence of oil, it was laced with gas and the gas smell was strong. I pulled the oil drain plug and drained the gas and oil from the oil pan, which I would estimate the oil pan being full of gas with the existing oil content.

I pulled the right bank valve cover and inspected the push rods for evidence of being bent they looked ok; I did not pull the left valve cover to inspect those push rods.

I don’t know the next move except to pull the timing cover and continue working through the inspection process pretty much an unknown situation for troubleshooting the problem. I am lost on my next move/my thoughts (bent valve, timing chain issue). Need assistance in troubleshooting the pragmatic cause of why the engine cannot rotate. Note: I took a breaker bar and socket and tried to manual rotate the crankshaft with no success. Help!