Speedos are not off by "MPH". They are off by a percentage. 3.42 --> 3.73 means your speedometer would now be correct for about 34" tires. So you are going 6-7% faster than you think (maybe worse because radials run short and bias tend to run as advertised).
You change out a gear in the tail of the T-case to correct the speedo, unless you have a newere VSS rig. But why not wait until you get those 4.56s?