The 300SL is the same transmission as a Porsche 928. Some 928 owners effectively use it as a 4 speed, skipping the dogleg first gear because it's hard to get to and very low, but that does not solve the highway cruising problem. Your best bets for an rear axle swap would be: 90 to 92 300E - 2.87, 93 to 95 E320 - 2.65, 92 to 95 400E/E420 - 2.24, early 500SL - 2.65. But then second gear starts are not likely to work very well.