help with obligatory girlfriend **CUSTOM** bike build....

Get a 44 mm or tapered head tube so you can use a tapered fork. Otherwise you're fucked for inexpensive options. There's the ebay one for £30 or something but just barely clears 28s i think, @bankiren had one on his kaffenback. That's 375 mm a-c. For 390-400 mm there's the kinesis dc37 or the condor fratello in the <£200 class and some aliexpress thing listed as available with straight or tapered steerer
http://m.aliexpress.com/item/32456133113.html