actually many heater hoses are 5/8" i.d. and so are most garden hoses; I
use a garden hose repair adapter with 5/8" heater hose to make up a
short adapter for flushing heater cores etc.  no reason you couldn't do
the same with a whole engine.  just remove a heater hose from its
fitting (difficult if they are old, but that may be a sign you need new
hoses) and attach a short length of new heater hose. on the other end of
the hose attach a female garden hose repair fitting, attach that to the
end of your  actual garden hose et voila.

If you are going to do this it might help to open the block drains in
your engine block and flush through there, then replace the block drains
and remove the lower radiator hose at the radiator (petcock will not
allow enough flow for a good flush)

good luck

nate