Exhaust manifold would be noisy but not smoke, head gasket would probably run worse at startup and then get better the longer it runs, the timing isn't adjustable (and any sensor that affects timing would turn on the SES light), and sugar in the gas tank just plugs up the filter.
My first thought is two-cycle oil in the gas. That will damage the catalytic converter if there's enough of it. What I would do is get 2 or 3 5 gallon gas cans and empty the tank into them. Your tank holds 12 gallons, so just whatever number you need. There's a fuel pressure test port on the line going to the fuel rail, just remove the core (just like a tire valve) and clamp a hose around it unless you have a fitting that screws on. It doesn't matter though, there won't be any pressure in the hose. Run the hose into one of the cans and connect a jumper wire between the 30/87 terminals on the fuel pump relay. This way's much easier and faster than dropping the tank. Afterwards, you should probably change the fuel filter, and then probably change it in a few thousand miles. Then fill the tank and see if it still smokes, it may take quite a while to stop. If it's still smoking after an hour or so of driving, it's probably something else and you
I had a car that someone put diesel in and it would start and die immediately, and there was lots of white smoke. This may be another possibility, I'm not sure what a smaller concentration might do.