This truck is so hard to love

Has a rebuilt engine with webber carb. It's tuned a bit rich and timing is set at 5 degrees. It blew a head gasket only has 10k since the overhaul. Called the shop and they don't believe engine had any issue. They believe it is my timing or tune. I know the z24 is infamous for head gasket but 10k miles is ridiculous. I don't know what I'm doing wrong also why is cylinder 3 and 4 especially super clean looks like it never ignited. Wires, plugs, cap and rotor are new. Do any of you know what could be causing this or could it be that the coolant washed the pistons.