It's worth measuring up to check, especially the clearance between kingpin and tyre, but if you stick with a similar tyre size you may be ok. IIRC the 5.5" rims are fitted with 185 section tyres as standard, which is also a good fit on a 6" rim. The other suitable size for 6" rims is a 195, ie only 5mm wider each side so you may get away with fitting those too. Personally I'd stick with the 185s, fitting overly wide tyres to a 4/4s means you lose a lot of agility.
