Yes, you need a v3.4 ASSIST to install v3.4.xxx SUP file (at least that is what Dell says for the 701N/705N - "You must also download the version 3.4 of the PowerVault Assist utility to perform the Operating System upgrade.")
As Dell still let you download ASSIST it might be worth seeing if it works with your Snap server -
http://supportapj.dell.com/support/d...7&fileid=48989
I think the web upgrade option was a later addition.