How do you know it has a 2.70 rear end ?
If comes out of the hole like a shot, then it is likely NOT a 2.70 ratio rear diff gear anymore, maybe a 3.50 or higher, which means on the highway you are revving higher and burning more gas.
Need to verify what rear gear ratio is in the car 1st, as it might be as simple as a gear change to get things back to decent fuel economy.

I can recemmend a great guy with a shop in Waterdown, ON (near Milton) called Vintage & Performance Restorations.... all he does is Classic Mustangs. 905 690-7637 and his name is John Miles. He has done lots of work for me on my various classic mustangs over the years.

I believe the normal auto trans differential ratio for 65-69 was 3.0 If you have your driver door nameplate still, it is easy to decode what it was born with.

Classic Mustang Dataplate Decoder