Few years back I had a 61 Bird with the factory 300HP 390 (only engine available in 61). Eventually put a 63 Tri-power set-up on it and bigger exhaust which really made it a fun car.

I really liked the way the car felt going down the road and the only changes I would have made were the addition of disc brakes and sway bars.

The big drawback on the 61-3 Birds from a performance standpoint is the weight. These old birds usually weigh well in excess of 4000 pounds. I have seen the HP figures on the 4.6s but have not come across torque numbers, and torque is the force that actually gets the car moving. Before I started taking the car apart and making any major moods to it I would check and see how the torque figures compare between an FE and the 4.6. Remember these engines are in a lot lighter car than the bird is and the lighter the car the less engine you need.

I'm not saying the 4.6 won't work, but I really wonder if it would produce the amount of torque required to move the big boat around like you would like it too. At a minimum, I suspect you'd want to use a lower rear end gear and OD tranny.

Admittedly, I'm pretty much into doing things the "old fashion" way but if it was me I think I would build an FE. Depending on exactly what I wanted the car to do and how deep my pockets are; 390, 428, 427 or stroker versions of these, and then check into using aftermarket FI on it.