I would use the Sales Part Cross Reference functionality.  This way you can have multiple cross reference numbers, for 1 sales part.  

Lets say you sell sales part number ABC to 3 different customers.  You could enter 3 different cross reference numbers (for each of the 3 customers), for 1 sales part number.  The sales part number is your number, for your product.  

Enter Cross Reference Data Explanation:

This activity is used to create cross-references between your sales part numbers and a customer's part numbers. This task is to be performed by a material administrator, a logistics responsible or an equivalent personnel.

When the customer assigns a name to a part that differs from that used in the IFS system, you can enter the customer's part number and description as a cross-reference. One sales part reference can be connected to many customer part references. The descriptions can also be different.

You can connect one or more sales parts to one or more customer's parts. This function is useful if the customer wants to use their own part numbers when they send you purchase orders.

This dual listing can be displayed on various documents, which makes it easier for both you and your customer to accurately maintain records using familiar part numbers.

This functionality is particularly useful in a repetitive/schedule environment when using EDI. Actually, it is required to establish a cross-reference in a multi-site environment, even if the part numbers in the sites are the same.

This activity can also be used to enable self-billing for a sales part.

Receiving advice information is useful for confirming deliveries made to the customer. This activity can be used to indicate the advice types that can be matched to deliveries of a certain part number, or to disable the possibility to match receiving advice for a certain part number. Receiving advice of type Arrived Goods only includes the arrived quantities and the date on which the goods arrived. The Approved Goods type is used to receive information from the customer about received and approved quantities, and the dates on which the parts where fully inspected.

In customer scheduling, you can create sales part cross-references directly in case the relation between your sales part and the customer's part number is nonexistent. The ability to create cross-references is made possible when a customer schedule agreement part is being created.

Prerequisites

This activity has the following prerequisites:

• The customer record must have been entered and saved.
• The sales part must have been entered and saved.

System Effects

As a result of this activity:

• The sales part record is connected to a specific customer part number that is used by the customer.
• Your sales part number can be entered on customer order lines, and the customer's part number and description will automatically be displayed.
• The customer's part number can be entered on customer order lines, and your sales part number and description will automatically be displayed.
• Both you and your customer can continue to use familiar part numbers since both will be displayed on documents.
• The sales part will be available for self-billing (When a customer order line is entered for this part, the Self-billing check box on the customer order line will become automatically selected, indicating that the part is available for self-billing).
• The receiving advice type will be set for the sales part. This setting will take priority over the receiving advice type selected on the Customer window. 
• The value in the Minimum Remaining Days at CO Delivery field in Inventory Part/Acquisition tab will be overridden by the value entered.

I too vote for sales part cross reference. We stock in cases, because that is how we manufacture - our boms and routings are in cases, etc. Our customers order in pieces, but we force them to order pieces in case multiples (except for VERY rare circumstances, because we do not crack cases).. but we have 1500 customers, and MANY of them have their own customer part numbers, so we use the sales part cross reference to allow a customer description, part number, and case conversion quantity.. Have done it that way since 2004.. Using sales part will cause a real proliferation of sales parts, make it very difficult to forecast..