Alright, it was mid 70's today so I took her for a nice drive and paid particular attention to what you've asked
because once you've driven your car for a while you don't notice stuff like this unless you really pay attention. Here is what I believe to be true:
1) With pusher fans, big radiator fan, extra venting & heat shielding, and ceramic coated pipes, in mid 70's weather you don't notice the engine heat at all. Zero heat comes through the firewall or trans area.
2) For pleasant cruising at highway speeds, with your wife, with whom you presumably want to chat with from time to time, anything above 2700rpm is going to be unacceptable because of engine noise (and I have specially crafted "quieter pipes" too).
3) If your car's engine is civilized enough to run without hiccupping at 2000rpm or just over, I would use that as your benchmark for the highway cruising speed that you expect to drive at, and then back in to your gears from there. If you plan on racing your car at speeds over 120 or so, forget about the .64 overdrive altogether.
4) Don't forget that you will also be going
slow, so don't make your gear combinatioin too steep in first gear.
EDIT -- BTW, I bet you only need like 50 ft/lbs of torque to cruise at 70mph. It's
how your engine runs at that RPM and whether you can lightly accelerate from that speed, at a given rpm, without it burping...
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