[v8volvo]

Interesting question. The AW70's overdrive would be nice to have for sure, and would probably improve the noise level and fuel consumption on the highway as you say. The gearing of the AW70 is probably similar (or identical) to its ancestor the BW55 in gears 1-3 but then it gives you an overdrive gear for cruising. So no real difference in gas/diesel gearing. Like with the manual M46, Volvo made its changes in gearing for gas vs diesel cars by varying the rear axle differential ratio, and kept the gearbox internals the same. True as well for the ZF automatic which was used in both gas and diesel cars. The valve body shift points might be higher for the gas engine but you can compensate for that to some degree with adjustment of the kickdown cable, at least in theory.

If it were an AW70L then you would also get a lockup TC, which IMHO would make an even bigger difference for efficiency than the OD gear would. 740s and 940s had the AW70L in the non-turbo gas models. Something in the back of my mind is telling me that the lockup transmissions never went into the 240s for some reason though. A look at the data plate on the side of the trans would tell you, if in doubt.

I think the answer of whether it could be swapped would depend most of all on whether the BW55's D24 bellhousing could be bolted onto the AW70. Things like the speedo and driveline could be solved easily enough but the first question is if the trans and torque converter can mate to the engine.