4.10 is the most common being found behind M40 and autos, but that gives you just 4.8% more, hardly worth it. Is your car a stock B20F K-Jet? They work best between 3000-6000rpm, 64mph to max speed in OD. Is noise your problem? Changing to an electric fan would help.
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69 142S Overdrive + 69 164S Manual

140's came with 3 choices of rear ratio--very early M41 cars (maybe 145 only) had a 4.56--later M41 cars had a 4.3--auto and M40 cars came with a 4.1 -- you would want to find a '72, '73 or '74 to make it a direct swap (maybe '71 too). Earlier cars used bearings and seals that are available as separate items (more $) -the later cars use the same parts as any 240 and are available as inexpensive kits. You can tell the difference this way -- early diffs have the handbrake lever at the top of the backing plate -- later cars at the bottom. The early diffs will bolt in -- but I don't think the later cable will reach the early lever. I went the other way with my '69 144S -- installed a '73 4.3 diff -- the early cable could reach the later "at the bottom" levers. If you want to get into internals -- a 240's 3.91 gears could go into your housing -- or further 240/260 choices of 3.73 and 3.54 -- a 3.31 would probably be way too long a gear.
I had a '68 144S on the road for many years with a self installed M41 - it had the normal M40 4.10 -- it was a good combo. -- Dave