The method I said (from memory) was the instructions of a "kit" with seafoam. As for hydro-locking the engine, the kit used a flow limiter on the line from the can, so I'm pretty sure they designed it so you couldn't have too much go in to hydro-lock the engine, but enough to foul out the plugs.
Right on the product it says it can be used in the gas to clean the injectors, oil to clean the inner part of the engine, and of couse the vaccume system to clean the intake and cylenders.
Some good images:
Seafoam - Mechanical Database
According to the site above, doing the same with water should work well too.
With 350k miles, I would be really careful with anything that cleans the engine oil areas, mainly since 350k miles worth of junk is built up, it could block oil pathways and kill the engine if they are cleaned and therefore put into the oil. On the flip side, if it has been treated within 100-150k miles ago, or the engine has always had high quality oil in it, go for the cleaning. This might be a really good thing for engines with sludging problems (newer toyota engines).