Having built several Proflex's and using Girvins the above options will work (using a combo of headsets CRC have been doing Cane Creek for bargain prices so is a cheap workable solution) However, one of my builds for a while back had 1 1/8" Frame and 1 1/8" Girvins which like you have pointed out narrows from the base of the steerer to 1" at the top section. the top part of the 1 1/8" headset had a shimmed split washer which was profiled. Insertion of this washer took up the slack between 1 1/8" and 1" with compresssion from the Stem and star fangled nut keeping it all together. The shim/split washer was approx 6mm deep. I am guessing that over time these shim/split washers ended up going missing or becoming damaged over time. I do have a spare for my 856 Flex but in all honesty go for a mix and match headset option. you can incidentally get a 1" crown race to sit on the wider 1 1/8" section on the steerer however not advisable!!!! Like i said CRC have been discounting Cane Creek headsets in both 1" and 1 1/8" flava, its a simple solution and will enable you to match your Girvins with your Flex's 
