Convert CMYK to Pantone

I have a client that gave me some artwork created in CMYK with plenty of half tones.  Unfortunately, it breaks down into 1558 colors.  We need to convert it to Pantone for screen printing.  The whole thing can be reduced to 6 colors without changing the look.  Is there a process that will automatically convert the CMYK colors to close matching Pantone colors without going through each individual color and halftone, which would take forever..

Thanks!