The closer ratio units are strongest. They can be very strong if you get a nickel input shaft (main drive gear) and cluster gear. I had a friend with a 68 Firebird that ran one of the 2.64 ratio boxes with nickel input and cluster gear and used to dump the clutch at 7,000 RPM with slicks and would run 10.76 without any breakage.
If the 7,000-RPM part has you baffled, that is because I forgot to tell you he had a 427 Chevy. Actually I was afraid to say because I didn’t want to cause any undue stress or pain for anyone. I can tell you that I did give him hell when he switched from his 428 to the Chevy.
Prior to installing the nickel gears, he did break the Super T-10's main drive and cluster, but after installing the nickel gears it held up and never broke again. The only bad thing about the nickel gears is that they are soft, which gives them their high-impact strength, but does not lend good long term wear characteristics.
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