There is usually more to it than just soaking via PB or whatever in through the Sparkplug ports.
Rust can be some ways up the Cylinder walls above the Piston's positions, and below them also, so merely getting it fred up and then starting it, it not so good, compared to cleaning all of that out, first...and, it is best to look into this via pulling the Head...and or pulling the Pan as well.
If you just got this Truck, you should do both of these anyway, no matter, and clean the Pan really well and correct it's usual 'domed' bolt holes and so on, and install with new Gaskets.
Removing and cleaning and inspecting the Manifolds is good to do also, check the warm up Baffle and see if it is frozen in some position, check how the Manifold ports align to the Head Ports...install new Gaskets, etc...
With rare exception, these are all things one should do anyway, in order to have the Engine in good shape.
Too, how did the Water get in there? Is there a bad Head Gasket? You will have to pull the Head anyway to replace that, and, if you have not replaced it before, you would do well to replace it now anyway.
I would say "Keep your 216' and enjoy it for what it is...no Small black will ever be as interesting or charming.
Plus, from what I gather, it is a huge amount of trouble and weird modifications and time and expense to fit a Small Block into these old Trucks.
