If your going to go EFI, the Dakota tank with the fuel pump inside is a very good option, but if your going to do a carb, I wouldn't worry about using the Dakota tank.
As far as the frame length, do you still have the 57's original frame around? By far the easiest way to get the correct wheel base is to maintain the 57's wheel base numbers. The biggest issue with using the cab , front clip and box is its way too easy to be off an inch or two. Things like how much space is between the cab and the box and exactly how the wheels it in the wheel opening can get you off a couple inches without even catching it until your assembling parts. Gene