You need a 5/8" combination wrench.------------Now look for a bolt with a 5/8" hex on it.---------There is only one and it is to the starboard side near the back of the lower carburetor.------------Loosen that bolt till you can spread the 2 arms to release the shift rod.
If ALL the bolts are out the drive shaft splines could be stuck in the crank shaft splines. Give the lower unit a couple of wacks with a big rubber mallet. Is the bolt under the trim tab out ??