Combines Is 644 6x38

Don't know if this is any help, but we converted our 844 into a 10 row 30" a few years ago. I know the "math" doesn't work on ours either, but we went with aftermarket plastic points for the entire header, bought 2 more row units, and made the conversion without any frame work whatsoever.

last year took a 644 and converted to a 10 row 20"

should be the same. 6 40 inch rows is 240 inches same as 8 30 inch rows. may be a little difference on fram supports underneath the head on the bar the units slide on. may also need to lengthen the hex shaft from each side. all in all should work alright. of course you will need to change the sheet metal in between the rows unless you want to cut the old to fit. good luck.