if it were mine, i'd pull a couple bearing caps and see what kind of shape the bearings are in.. if that thing got riun very long with lots of coolant in the oil it may very well have trashed the mains or rods.. (DT-466E's.. Can and do throw rods if run with bad bearings)..
Next, if the bearings are still good, i'd do a mini-inframe on it.. replace all 6 liners and pistons and of course the head gasket. and here is why.
thats assuming its leaking at the liners.. when you pressurized the system was the water running down from inside the liner and dripping or was the water running down from outside the liner..
if its from the inside you might be able to just put a head gasket on it and be done.. *IF* everything else is good and disturbing things doesnt cause a leak..
from the outside? its a liner seal.. and i'd change all 6.. along with the pistons and rings.. (rings at the very least)..
back to why change all 6.. when things get disturbed after being tioght for many years is when slight variations occur in reassembly.. a slight difference in pressure on one ofthe other 4 liners and then it decides to leak 3 weeks from now. a liner seal without swapping the liner.. can you ensure 100% it seats exactly back down where it was so the rings dont "run the ridge" on the liner.. obvipously a lot depends on your budget and what the intentions are for the bus.. if the intentions are to make a new skoolie and drive it forever then i'd probably overhaul the whole thing..(bearings, valves, pistons, sensors, liners, gaskets) if the intentions are to make it run and flip it then i'd probably do 6 liners,rings and a head gasket.. if the bearings are shot you pretty much should do those.. you could then sell it as a refreshed engine.. #6 is always the worst.. sounds like an over-heat event for sure.. the computer would tell us just how hot it got... it stores the last 3 overheat events.. a REAL hot event is going to have resulted in the head being warped.. becomes a toss up of whether it will seal on a new gasket... if the leaks are liners and not the gasket now then a real good chance the head will seal up against a new head gasket..