lecoq,
Instead of whimping out your T-bird,
how about we go more efficient ! If you get more power out of the fuel you will use less fuel. A carburator is great on race day but not so much in traffic. fuel injection is better, more precise.
Ford FE EFI System [FORDFE] - $3,994.99 : Mass-Flo EFI, The World's Only True Plug and Play EFI System.
The link above is MASS-FLO, they make Multiport Fuel Injection systems that work for older muscle cars. I can vouche for these guys, I have this system on my 67. Better fuel management and performance with no cold start problems.
The link below is an article on the 700R4 4 speed transmission being fitted to a FE 390. It's what I have and one more gear takes away 900 rpm on the highway at 70 MPH, big improvement. Hughes Performance made mine, They can probablly make you one for that 429.
700R4 Transmission / 390 Ford Engine Combo - Custom Rodder Magazine
If you like what you see here's the link for Hughes Performance:
Welcome to Hughes Performance
Give them a look, I think it's a better way to go and will improve the value of your T-bird because you can keep the original trans to sell with the car if you ever do ( they didn't want my old C-6 trans when I purchased their 700R4). There are other modifications to make with the trans swap though; the drive shaft will have to be shortened and trans mount location need to be moved. If you have mechanics as you say, they can do it.
Think about it before you go and desecrate that beautiful work of art in your driveway.
Good Luck,
Lad