I'd get the '54. Likely better motor, the tires and 1-piece rims are a HUGE plus. You can do your own brake job to save money, but you can't build your own tires/rims. You did say the rust is minor, and you can drive it home! I don't think anything could be more satisfying than driving it home the first time. I wish I could have done that with mine.
You may find a dump bed to be super useful as well. You could turn it into a flatbed with removable lower-profile stake sides (that's what I'm doing).
Does the 6400 have a 2-speed rear, or just the 3-speed aux trans? Either way, I'd be very jealous of your overdrive. I may do a motor swap for more power someday, but that won't make it go any faster if I keep the RPM's in check...
