There are not many drw with 3:73s... There out there but most will have 4:10 anf if I recall right you may even come across some with 4:30s and the 450s I wanna say have 4:88s
I believe ( and someone correct me here if I'm wrong) the 450's from 99-03 were all chassis cab trucks which have a narrower frame rail setup out back.
I don't think a 450 axle would be a straight swap into an F250-F350. Now in '08 they Ford started the F450 pickups with a regular dually bed and those rear axles may be swap into an F250-F350 frame but I'm not 100% sure about that.
I would love a set of custom 3.90 gears. My 4.10's kill my mileage, and the 3.73 is just to tall for my pulling needs. Maybe I'm just daydreaming...
The wife towed this last Saturday while I rode shotgun and didn't have any problems at all. I've never towed with a dually so I guess I really don't know what I'm missing. I sold my 30' TT about a month ago but when I did tow with it my SRW had no problems in stability or sway but I have always wanted a dually and with just going with this kit I think I would be better off instead of a whole axle swap. But that price is a little steep for me right now.