re: "...There apparently was a significant change from 79-80...."

You didn't mention whether you're talking about automatic or manual transmission. If the former, then you're forgetting about a change from '81 or '82 to '83, whenever the AW70 replaced the BW55. Getting that four-speed automatic which actually has an overdrive (~0.8:1) for the top, fourth gear calls for a drastic change in the differential ratio, compared to the BW55 with a direct (1:1) top, third gear.

Also, isn't the 240's rear axle *very* different from the 140's concerning various linkages (trailing arms, transverse rods, etc.)? I know the '75 164 used the "new" 240's rear axle (different from the '74 and earlier 164), but I thought that the 140's axle is quite different. How are you going to position and support a 240 axle on the 140's chassis?