Yes! Sound like an good approach.
I recommend you to read the following document, as it include all the necessary information about C, dll and LabVIEW:
Using External Code in LabVIEW
http://digital.ni.com/manuals.nsf/web_productcurrent/4F1447F7CD83D6D88625690D00637CED?OpenDocument
Also, check Keithley website. I checked and it seems that they have drivers for your data aquisition card for LabVIEW. Click the "Software Download" link:
Model DAS-1600/1400/1200 Series High-Speed Analog and Digital I/O Boards
http://www.keithley.com/PRODUCTS/modelNumber_tmpl.jsp?page=main&productType=pci/isa_plug-in_boards&modelNumbers=DAS-1200,DAS-1400,DAS-1600
Best regards;
EJV
www.vartortech.com