Sad Story On a Gear Vendors OD

But with that low of RPM? They must be using an OD to get there MPH?

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I'm not fully getting this. or how you mean it in your reply.

I wouldn't be in a OD gear up hill unless the rpm was high enough to support the move. OR, not draging the engine down to low rpm wise.
What Ford OD did you toast? The one that came with the Fox Mustangs? They are low rated in torque handling.
Historically speaking you say. Well, you should take that out of your worries since you know about it and can find out the strength of any transmission and avoid a weak one. So now your fears are unfounded unless you get a weak transmission. Then the fault is your own. All the older transmissions are lowin power handling.
I'm pretty sure Passon has torque ratings to there stuff. Likewise over at Silver State Transmissions.
Reguardless, the torque ratings of the tremacs or other transmissions, you should be available to find out what they can handle torque wise, in foot pounds.

Do you have a need to travel at 160mph? Then gear accordingly. IF you think you can tow with the gear ratio needed for a 160 mph blast, I think your only fooling yourself and laying blame of the crushed dream on the transmission. Before looking at a GV as "The only choice" , an investigation is needed.

OH, 6500 is NOT a relativly low rpm. that would be 5K or lower. FWIW, the factory rated the small block to 6,000 - 6,200 rpm as a max rpm because the oiling system may not be up to par for such rpm duties. The problem is mostly in the oiling passages not always being the same size and failing when pushed beyond the suggested rpm range. Others here (Myself included) have went beyond the suggested rpm factory red line just fine because the oiling passages were larger than normal. I have had them fail as well at 6000 shifts.

This engine limitation also applies to transmissions. Know there available torque capacity as the factory states it and use that as the red line max that you don't want to approach never mind exceed even though others may have exceeded it often with success.