Your best bet is to put your street name in a separate field in the Customer table from the very beginning instead of combining it with the street number. That street name should then be a drop down list or pop up menu of street names from your 2nd table to make sure that data entry/spelling errors don't keep the records from matching.
You can try setting up a calculation that extracts the street name from your current field, but this may not work for every address entered. In most cases, you can't just use a calculaiton that omits the number at the beginning as there could be an apartment or suite designation following the name of the street. So any calculation that you come up with will be at the mercy of how consistently the address was entered in the first place.
You might want to use a partial text matching search portal to select the street record to link to a given address record. here's a demo file: https://www.dropbox.com/s/0pm1gdqcfi2ndpv/EnhancedValueSelection.fp7
It's in the older file format so if you are using FileMaker 12 or newer, use Open from FileMaker's File menu to open the file and convert it to the newer file format.
That link you sent me provided the perfect solution. Thank you. If I could pick your brain for one more thing, is there a way to write in script to copy just the first five or six characters in a field?
I believe that others have already answered that question in a new thread:
Left ( YourTable::Field ; 6 )
will refer to the left 6 characters of YourTable::Field