Anything is possible, but a 6 row would be just about right for the 42. 8 rows might be a little much. MIght be better off going to 12 row narrow planter with a 6-30 headIJ
I ran an R50 with an 8-30 cornhead for several years with no problem. The 42 and the 50 are similar in size so I don't think you would have a problem. In 200 bu. corn it would pull that R50 down under 3 mph. but it makes for less driving than having a 6 row. In 100 bu corn we ran 5.5 without even working it or overloading anything.
I wouldn't even consider it! I run 135 bushel corn and my R42 is "plenty busy" with a 6 row narrow at 3.5 to 4 miles per hour, and that's with the mod's! I'm not complaining; - I'm extremely happy with my Gleaner!