For fanned designs, you need to get a pickup built for one more string than you have. So 7 fanned requires an 8 string pickup, 8 string fanned requires a 9 string pickup, etc. Of course the Alumitones are just measured in inches, and your builder probably has a good idea of the right measurements. I put a 4.0 in my Agile and had to expand the routes a little bit, they were more like 3.8". But yeah, the 4.0 is probably the right choice as the 3.5 is just a bit on the narrow side.