Hi Dave,
I'd like to support the recommendation of a Weber carb, but would like to propose a smaller, cheaper version - the Holley-Weber 5200 progressive 2 barrel.
I also have the 250 engine and the C4 automatic tranny in a '73 Maverick.
I bought my 5200 from Tom Langdon at Stovebolt Engine Company. Tom's company caters to inline 6 Ch*vy's, but he has been great to deal with for my Ford 6.
The stock Carter RBS is rated to flow around 170 cfm - the H-W 5200 is rated to flow around 215 cfm - a 25% increase.
A rebuilt 5200 from Tom runs around $85 with an electric choke. Tom also sells an adapter to go from the 2 bbl carb to the 1 bbl manifold. The adapter was around $15, if I remember correctly.
The primary barrel of the 5200 is smaller than the barrel of the Carter, so you can expect better fuel mileage if you can keep your foot out of the secondary. I can really feel the kick when the second barrel opens - it's a lot of fun!
I had to do a little fabrication to get the throttle cable to connect up to the 5200 - but it wasn't too challenging.
I am actually working on my own adapter and a new throttle connection, but I successfully used the adapter from Tom for over a year - and very happily.
If you talk with Tom beforehand, he will ensure that the jetting for the carb and the throttle connection are correct for your car - he did so for me.
Regards - and good luck,