If looking at a 4.10 vs 4.30, I agree... less than 5% difference, and probably not worth the effort and expense. But there were some early 140's with the 4.56 rear (I think we're talking about the B18/M41 combo here, for the most part). So it might be worthwhile to find out exactly what the guy is selling. Many are illegible after so many years, but all diff's originally had a tag on the exterior of the housing that shows the gear ratio as assembled at the factory. Some of these tags just have the ring/pinion gear tooth count (41/10 or 43/10), while others will have the resultant ratio (4.10 or 4.30).

I also agree on the gearbox comments, as there is nearly 9% difference between the early and late (73 up) 1st gear ratios - 3.13 vs 3.41.
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Gary L - 1971 142E ITB racer, 73 1800ES, 02 S60 T5
BlueBrick Racing