Ryan,
A number of years ago I built a log splitter and learned a number of important points.
First you need a two stage pump that can develop about 3000 PSI. The hydraulics on the 318 are only about 900 PSI. A pump from a tiller will probably also be a lower pressure pump.
Second the pumps are normally direct coupled to an engine. Belts will slip on a high pressure pump.
Third the standard splitter cylinder is a 4 X 24 which will require about a 3 gallon tank for supply and cooling the oil.
After building my own, I would recommend just spend $1,000 and buy a brand new 20 ton splitter on trailer wheels with a 5 HP Honda engine from Northern Tool. Paint it green just to confuse people.
I am older and a little smarter now.
George of Buford