The coloured, plastic toothed gear is selected to suit the rear axle ratio. The number of teeth vary but they all work on the same spiral gear in the box. Using an OD in a car with a 4.10 axle means that you will have some speedo error due to Volvo never using the OD with the 4.10, therefore there isn't a "correct" plastic gear. However, as you are using a J type OD, something that Volvo also didn't use in the Amazon, you might be able to get something suitable. The drive gears for the D type are very hard to find in the UK, even the OD specialists don't have any. They are being remade for the J type but not the D, which isn't good news. Are D ones available new in the USA?

I have an M41 built into a M40 case, not ideal for oil checking, with a D OD with a 4.10 in my single Stromberg 121 4 door. Makes for nice easy cruising between 70-80. Rather quieter than with the D OD in my wagon with the 4.56, but that has a B18B as compensation.