I also used some special lock washers that I found at fastenal, They are a 2-piece washer that is made to slip in one direction and grip in the other, they really seemed to work well, we'll see how they do in the long haul.
Are you talking about Nord-lock washers? If you are, there is no need to worry about them coming loose. They simply dont. I've been using them on forklifts for 12+ years.
Nice work, I first considered a 6bt swap in my 94 E150, before changing my mind to a 4bt into my Explorer swap.