It should handle a 5 footer just about anywhere you want to pull it. On hilly ground with steep slopes, cutting heavy brush with a heavy duty 6' mower, the L3400 will struggle a bit. On flat ground doing light mowing it will possibly be OK.
I'd talk to my dealer and explain that you really want a 6' mower but you're not sure how your tractor will handle it on your property. You might talk him into giving you a trial run.