I agree with Evan but I'd go with 4.56...with a little hesitation
With a 2.66 first and a .50 6th gear that's the only rear end ratio I can see working anywhere close to right. Unless you could find 4.71 gears. Those would be closer and once upon a time there were 4.86 gears for the 9 inch. 4.86 would be almost perfect in my opinion.
You can use the Tremec calculator (or there are about 100 other ones out there). Here's Tremec's:
https://www.tremec.com/calculadora.phpPlug in your desired rpm (I used 1900 and I believe that is an okay target in your case--2000 would be better for hills and wind), your tire diameter (I used 28), your axle ratio (I used 4.56) and the ratio for every gear in your transmission (you already posted those). With all the factors you mentioned and using a 4.56 rear at 70 mph you'll be turning 1900 rpm in 6th. That in my opinion is the lowest rpm you need to consider with a carbureted 350 and an AD pickup.
If you used 4.86 gears with 28" diameter tires, your rpm at 70 would be apx 2050 and at that point you're right where I think you need to be. However your starting line ratio would be 12.92 and that might be more than you want. However the original AD starting line ratio was 12.08, though so you are not too far off (that is in case you ever wanted to use it like a truck). If you don't care about using it as a pickup your ratio could run down to 10.5 or so. If you plan on putting 1000 pounds of "stuff" in the bed and driving without burning up your clutch, you ought to at least have an slr of 11.
If you want more reasons I think this is what you should use, please let me know.
