Alright, how many watts do you really need?? If you need 3500 watts, you need to reconsider running the dozen or so server racks that you must be thinking about.
It is more like this: 3500/120 = 29 amps. And a typical inverter is something like 80% effecient. You would have to check on the inverter to see on the actual current draw at maximum draw. In fact, the inverter may actually come with the fuses and wiring you need. I know they have big inverters like this in tractor trailers (microwaves, min-fridges, etc.), so there has to be a solution.
I don't you really need more than a 500 watt inverter. And of course, you can always fuse it at a level you feel comfortable with; lower isn't going to hurt anything.