I can't say anything about a 1040 -- never heard it mentioned around 240 cars.

A list of differences between 1030 and 1031 (notes that I have collected over the years) are that:

A 1030 is about 10.125 inches from the rear cover surface to the front of the pinion neck. A 1031 is almost exactly .5 inches longer.

A 1031 Volvo housing will NOT accept the TrueTrac LSD due to slight dimensional differences from the 1030.

The 1031's have the "ears" on the right side of the pumpkin (they are not machined on 240's. They attach to the torque tubes on the 740 suspension).

A 1030 has a single gusset on each side of the pinion neck, roughly centered along the pinion centerline...
A 1031 has 2 gussets on each side; with the larger gusset spreading back from the top of the pinion neck; and the smaller gusset following the pinion centerline.

The differentials are all the same up until they changed for the speedo tone ring. The only difference between a 1030 and 1031 is the housing itself and the R&P (ring and pinion). The differentials can be swapped back and forth. The R&P's cannot.

The later model cars (86 and up) with speedo tone rings are, I believe, all 1031's. The true trac units that will be put in a car that requires a speedo tone ring will be different only to fit the tone ring. You can still swap tone ring or no tone ring differentials from 1030 to 1031. Only problem will be that you won't have a working speedo if you need a tone ring and install a diff that does not have a tone ring.

The pinion bearings are also different from 1030 to 1031. So make sure you get the right pinion bearings for whatever rear end you have.