Myself and a Mechanic cannot diagnose timing related issue in car

I replaced both valve cover gaskets and all 6 fuel injectors. After replacing
the rear valve cover gasket and the 6 fuel injectors car ran but did run a bit
rough. I replaced the front gasket and then after putting car back together it
would attempt to turn over but never catch and start. I have replaced the
starter (Thanks for lifetime warranty from autozone), this did not fix it. I
replaced both crankshaft sensors. Compression test was fine but when
mechanic was starting the car with the front bank of spark plugs out he saw
flames that had come through the exhaust manifold. Drained and replaced oil
and all spark plugs have spark. Mechanic thinks this is a timing issue, I still
believe I have a ground that is not fully grounded but I have checked what i
believe to be all the grounds. Fuel injectors seem to be flooding engine but
they are new, may have a couple problems but please throw ideas out!