the 283 heads may not have the holes in the front for alt., power steering, a/c brackets,,
and they wont have the 7th exhaust bolt hole for your manifolds,,
and you would need a different flywheel..
the 283 is a good engine but will have a lot less torque than your 305 and none of your accesories will go on. you will have to use headers because the dipstick hole on a 283 is on the driverside and will be covered up buy the stock manifolds and 283 manifolds have 3 bolt flanges compared to the 305s 2. the early 283 manifolds wont clear the frame. the heads will need hardened seats installed to live with todays gas if not already done. as i said earlier a good engine but a poor choice for this era car
At best you'll gain only 1-2 MPG with FI over a properly running CCC Q-jet. It will take a long time to recover the expense. You can get a well built CCC carb from guaranteed carb, Mountain Man ( Don't know if he does CCC?), Cliff's Performance ( $$$) and others. Expect to pay around $300- +/-. Mileage will depend on your gears and how you drive, but I would expect 17 city and 19-20 Hwy. Your mileage may vary.