The truth?
With a properly tuned carb you'll get better mileage and power from a carbed induction engine.
I have done a Magnum conversion into a '68 Dart and it's a bit of a hassle, but the TBI set-up is not really any harder to do, just less powerful and more finicky to work with.
I've owned and driven Magnum powered vehicles and their fuel mileage really isn't the greatest, but the power over the TBI engines is quite noticeable.
The TBI engines do seem to do better, mileage-wise, however.
If I were going to do a V-6 or V-8 TBI conversion, I'd use the '90-'92 360 set-up, or the '90-'91 318 set-up, as the wiring is a bit more simplified over the pre-'90 set-up.
Mark.