Hy ykp53, welcome, well that is a dilemma for sure! I am not 100% sure but I think the 6400 might have a 261 in it and the 4400 would have a 216 or 235. In any event the 54 will have a precision bearinged engine. If you have the space and facilities I would be inclined to buy both of the trucks and build one out of the two and sell what you have left. As you have seen by the bill the cost of the brake system overhaul is expensive and you will probably have to do the same to the 54 if you buy it alone. I realize you only wanted a truck and not a project, but that is my best advice, hope it helps.
