Depending on what wheel base the Dakota is, you will probably have to take a section out between the wheels. The area right in front of the gas tank is an easy place to shorten the Dakota frames. You will probably have to doc the ends of the Dakota frame as well. Most have to be trimmed just behind the rear shackles.
That truck looks pretty sweet, I suspect the floors are probably in pretty good shape, but a lot of guys are using the Dakota floor pan and lower firewall and attaching the floor to the older cabs. Then everything attached to the Dakota firewall fits. The Dakota cab is about 6" wider (measure yours) then the 50s cabs, so you have to make some adjustments for that.
I love those 57 Dodge trucks. If I ever get done with my current project, I would like to build a 57. I've done a 50 (48-50), a 52 (51-53), a 54 (54 & early 55), a 56 (late 55 and 56), and a 58 (58-60), and my current project is a 39 (39-47). I have most of the 60s, 70s and up to mid 90s covered as well. Missing is the 57 only body style and the 61-64 style. Gene