If you are running an 8 3/4" rear and the 4.10-geared 3rd member is ready to go I'd just put it in and see how it runs, doesn't take more than an hour or 2 if you know what you're doing. If you don't like it then get some 3.55s.
Just a ramble about gearing... Around here the freeway speed limit is 75 and I like to cruise 75-80 which puts my Duster's 360 just under 3000 RPM with 2.94 gears and 28" tall tires. I plan to put in 3.55 gears because that's what my cam was custom-ground for but then I won't feel so eager to cruise at 80 mph. I also go to events at a track about 160 miles from me so my solution? I got a 1972 Dodge D200 to tow it. 17 MPG on the freeway is what I get currently with the Duster, it'll only get worse with 3.55s. No I don't think I'll save any money on gas in the long run (I doubt the truck will do better than 15 MPG when towing) but I really really don't like driving a car for 2+ hours with the engine at 3500 RPM. The biggest reason though is so I don't have to worry about breaking my Duster at the track and not having a way to get home unless I pay out the @$$ for a 160-mile tow.
Not sure if that helps at all lol just what I'm currently dealing with regarding gearing on my car.
