Just use Excel.  Optional fields and pricing have to be separate sheets.

I'm sorry if my wording was a bit confusing, I should have said Optional fields sheet.

For one item using the native excel import option, our Item_Tax_Authorities sheet uses 94 lines and our Item_Optional_Fields sheet uses 42 lines. That would total to about 188,000 lines of Item_Tax_Authorities and 84,000 lines on the Optional_Fields sheet. 

For each item we only actively need to import around 12 lines worth of data for each. I was curious if their was a field matching module or the like available or perhaps if someone had something that they had tried.

Just add only the optional fields you use to the sheet.

Once again thanks for the reply! We have tried doing that and other variations and it just throws back errors regarding those sheets if we skip those lines.

We're looking for someway to automate this process as this would be very time consuming.

Is it because you have mandatory optional fields you need to change as optional so you don't receive the error?  Alternatively get a dev to write a small item importer tool that imports via the Sage 300 Web API using json packets, or create an Excel macro or even Sage 300 Macro that takes your source data and transforms it exactly how need - meeting the optional field requirements for each item is easy with a macro and some trial and error.  The investment in time up front to build an automated solution will be worth it!