I think the different questions from the kit car company and the cam company relate mostly to your cruise RPM. You want the engine to be happy at cruise, not "lumpy" like it may be at idle. So, you need to figure out what speeds you will be most likely spending the most time at, and what size tires you will have, and then pick the gear ratio and cam from there.
For example, let's say you are going with the toploader, which is a 1:1 final drive. You have selected tires that are 26" in diameter, and you figure your cruise speed will be 65 MPH. Your engine RPM is calculated as (MPH X Gear Ratio X 336)/(Tire Diameter). If you plug in 65 MPH, 26" tire diameter, and a 3.50 gear ratio, you will be at 2940 RPM at cruise. Thus, you can pick a pretty healthy cam, because most cams will be running well at that speed.
On the other hand, let's say you will run with the TKO with the 0.64 overdrive and the 3.50 gears, a 28" diameter tire, and your cruise speed is 55 MPH. Then, your final drive ratio will be 3.5 X 0.64, or 2.24:1. This will make your cruise RPM 1478. A high duration cam will NOT be happy at that RPM, and will make the car run poorly at cruise.
These are just examples, of course, but these are the kind of things you need to think about, and why the kit car company and the cam company are asking these questions...