don't do what you said to do you could smash valves or possibly cause fires ... bad things might / will happen. Get your cams and crank lined up first before spinning anything much less firing the engine.
1) use a gauge (not sure the name but typically they are a dial gauge that measures in hundredths or thousandths of an inch.) mounted to the deck of the block and spin the crank till the proper piston (whichever should be up depending on your engines shop manual) is as high as it will go. (this could conceivably be done through a sparkplug hole)
2) look through the spark plug hole and eyeball it till the proper piston is nearly tdc (you may be off a few degrees but at this point some marking on the crank should be pointing up, typically a dot on the timing cog)
3) set the crank so piston one is full compression stroke (or between exhaust and intake strokes) set the cam to this position as well. You will need to visualize the cam turning and opening and closing the valves to make sure the rest of the firing order works correctly. if you spin the engine with a ratchet you run the risk of striking a valve. also check the piston that is between the exhaust and intake strokes has the proper valve overlap then belt it. You may need some other measuring equipment to make sure the cam is opening and closing valves at the optimal times though.
Just to reiterate: do not try cranking the engine or firing it till you get this all sorted out. and if you spin the crank with the head on that can damage your valves if it is an interference engine or a non interference engine with aftermarket flat top pistons, bigger cam profiles, etc.
to address your comment. If the cam is 180 degrees off (360 deg for the crank) it should not make a difference. because each 360 the crank does the cam does 180 meaning piston 1 will be tdc when the cam lobes are pointing up and again when they are down.
but you still need to make sure your cams are at exactly their 180 degree spot relative to the cranks 360 degree spot. You can't set the crank to 0 deg and the cams to 45 deg this could end very badly, possibly even while you're torquing down the head.