As mentioned an 843 is already a 30" head.

Making an 844 (4 row wide) head into a 5 row 30" is very easy and cheap.      The frame will work with no modifications.

Making it into a 6 row 30" is going to require cutting and welding of the frame, auger and auger trough.   

Very likely won't be worth the time spent vs buying an 863.