Yes, you will need a trunk, support brackets, interstate fender struts, a piece of wiring harness, and you'll have to convert your bag lid hinges to interstate cable ties since hinged lids hit the trunk after about 4-5 inches of travel. And if your bike is a 97-98, you lids will not have the internal bosses to attach the cable ties to. Something can be fabricated (to the fore and aft latches), or you can forgo the cables and set them on the ground (you'll still have to switch from hinges to latches) (but at some point one may fly away and get run over by a cage). And then there's painting the trunk (unless you get very lucky, or have a black bike). And she will probably want passenger floorboards.
Now, all other things being equal (miles, condition), a used interstate runs about a $K more than std/tourers, and guess what? The conversion will almost certainly cost more than a $K (and a bunch of work and shopping and shipping). Of course you will be hard put to find an interstate with your low miles, but if you make the swap you also get a 7 gallon tank and a radio and gas gauge and a fairing.
Or, you can go with one of the cheaper, smaller trunks which can be modded up to fit on your luggage rack. Like these.
http://store.mutazu.com/One other thing that I have heard (but no experience) is that the Mustang (Vintage Contoured) sissy backrest pad for Valk is wider and more curved than the OE or Ultimate or others.
http://www.mustangseats.com/Product/75991/Vintage-Contoured-135-x-95-Sissy-Bar-Pads-Honda-Valkyrie-No-Studs 
