All these suggestions are good.
One more thing if I remember right, it's been many years, is that the N injectors have fuel port that are staggered where the HV injectors are inline if you understand my terms. This would suggest that besides the problem of the arms not being lined up you would also be needing to replace half or maybe all the fuel lines under the VC!
And just remember that those two valve engines were ancient history in the 1970's. The fuel lines have probably be on and off many times and they will be prone to cracks in the flared ends.
So even if you do get the engines running from the water in fuel you will need to keep an eye out for fuel in oil!
Did find this:
Rebuilt HV6 Injector for Detroit Diesel Engines No core fee.
www.adiesel.com
Only $130.40 with no core charge. Have no idea if they are any good but sounds like this company might not care, just get it running.
I'm a little worried about where this "tug" is operating hope it's not near a bridge I might be driving across or a ship I might be riding on as it sounds like it could be a shady operation.