Not trying to split hairs but E85 is closer to 70% the energy per volume.
Wikki has some equivalency tables so take them with a grain of salt.
http://en.wikipedia.org/wiki/Gasoline_gallon_equivalent

Given that you have rated E85 by it's average RON and MON numbers I wonder what the equiv would be for 98 Oct give it is usually reference by its RON only (and MON is always lower). I believe that E85 (or was it pure Ethanol) has a RON of around 100 but am not sure where I got the info from.
http://en.wikipedia.org/wiki/Octane_rating has some ratings

Either way, watch rubber components as E85 loves to eat them.

BTW always be carefull when welding old petrol tanks, I wouldn't recommend it unless you know what you are doing, better of getting a pro to do it, then have it blow up on you.