There isn't really any right or wrong way to renumber steam locomotives of the same class. There were a few approaches on real railroads if that kind of information matters to you.
As most steam locomotives were purchased in rather small blocks with some notable exceptions, one of the most common methods was to have the first locomotive of that class numbered as the lowest number of that class of locomotive and then consecutively numbered higher. This works well when you have perhaps 5-20 locomotives in the same number class. ATSF is a good example of a railroad that referred to it's locomotives as the "2900 Class for one type of 4-8-4s as an example. They would later have a different class of 4-8-4s referred to as the "3751" class.
Alternatively there were railroads that used almost random numbering systems for steam locomotives. In the case of the PRR, there was no real system for how the 425 K4 Pacifics or the 598 I1 Decapods and other locomotives were numbered other than using the numbers that were available. This was in large part due to the total number of locomotives on the railroad and numbering consecutively would have resulted in 5 digit numbers. The railroad instead opted to reuse numbers for retired locomotives as those numbers became available.
In short, its your railroad and I'm sure you can come up with something interesting to tell you visitors as to why your fleet of Postwar Lionel locomotives all have unique numbers.
The real question is are you going to find heat stamps to come up with all these new numbers and confuse future collectors for years to come! 
