I know what Derek Uk means but he has it backwards. To lower the rpm at speed you need to go to a LOWER NUMERICAL axle (often referred too as a HIGHER ratio). The 1980 BW55 has a 3.9 rear. You could go to an easily found later 240 auto which would have a 3.73. Later M46 equipped cars have a 3.31 (probably way too long for a 103hp 2.1 litre). Volvo 260 series with BW55 auto have a 3.54. The 3.54 ratio gives you 20mph per 1,000rpm -- probably as "long" a ratio as OK with the B21F.
I think what you need is the more radical solution of installing an AW70 or 71 (essentially a BW55 with the overdrive drum at the front). With the current 3.90 rear the 2.1 motor will keep its spunk in the lower gears and the 30 percent reduction in rpm will soothe your nerves (although the B21 can stand up to high rpm all day) on the highway.
At a minimum you'll need the AW70/71 & torque converter (should always go as a set) and drive shaft (mark it) from the donor car - take the electrics too if you want to keep lock-OUT an option. If you find a '93 AW70L you absolutely have to take the converter for the lock-UP feature to work. - Dave