Your state law has the last say on that whether it requires your OE seatbelt to remain in place, whether the seatbelt has the required NHTSA Letter of Approval, is DOT Compliant, etc. but my main point is going to be this... even if you install a 4 or 5-pt. harness, keep your OE 3-pt. seatbelt installed.
Why? Because 4 and 5-pt. harnesses are such a PITA to use as well as being so uncomfortable for street use that you'll soon be back to using your factory seatbelt which takes, what, 3 seconds to connect? Even my simple 2-pt. cam-lock 3" lap belt I use when I'm offroad is too much of a PITA for street use.
One thing for sure, only push-button latches have any chance of being legal so if what you're looking has the cam-style latch, it has no chance of being street legal. But the cam-style latch is BY FAR the best for offroading since it will work even if it gets dirty as happens in an offroad vehicle. I wouldn't even consider buying a push-button style 4 or 5-pt. harness for offroad use.