Back in the later '60s (and probably earlier), the prevailing orientation was to gear the vehicle for towing such that at highway speed, while towing, the engine rpm is near the peak torque rpm for the particular motor. In those times, depending on the motor (the Chrysler 273 2bbl had a peak torque rpm of 1800 rpm, yet other V-8s usually were in the 2800-3200rpm range) the peak torque usually happened in the 2000-3000 rpm range, yet in more modern port fuel injected engines, the torque curves are much more flat instead of being a curve that has a distinct peak at the appropriate rpm.
In the earlier orientation, the Valiant with a 273-2bbl would work fine with the stock gearing, but a 383 or 413 or 440 (whose torque peaked in the 3000 rpm range) would need lower gears than normal. This was all laid out in a Q&A section of a Motor Trend column back then.
Not being sure of what your transmission might be, do not run it in a lower gear than direct (high gear that's 1:1), unless it's a temporary situation going up a hill or similar. Change the rear axle ratio, even though this might affect fuel economy when not towing. The original overdrive automatics recommended not to tow in overdrive too, but with electronic controls being what they now are, you might try it and see (with the electronics putting things where they need to be for efficient operation).
In the case of a carbureted engine, knowing what the manifold vacuum is can help, but the response of the engine to additional throttle input can be an indication too. If manifold vacuum drops below about 6" Hg, the carb will be running in the power mixture and fuel economy will decrease significantly, but if the vacuum stays above about 12"Hg at cruise, then no significant amount of extra fuel (to feed the extra load) might be used. Basically, if the engine's not near the peak torque rpm or in the meat of the torque curve at highway speed while towing, the manifold vacuum will be lower (and all that that entails) and the engine will be working harder under the additional stress of acceleration/hills than it should be.
Also, using a smaller engine to tow with can be less fuel efficient and less safe than using a larger engine. For example, some friends in our Mopar club used a 1/2ton four wheel drive extended cab pickup to haul an enclosed trailer (with a Road Runner in it) to Mopar Nats one year. Going through the hills of KY, they would barely crest the hills at 35mph and then hit 85mph at the bottom to make it up the next hill at WOT at 35mph. Although it was an engine in the 360 cid range, it got terrible fuel economy AND it was rated to two that much weight by the factory.
When they got back, that 1/2 ton got traded in for a HD 3/4 ton four wheel drive extended cab with a 460 V-8. Next year, same loaded trailer, same route, they just set the cruise and let it roll and got better fuel economy with no drama in the hills. No drama in the hills meant a safer trip for everyone on that road too. Plus the heavier truck had a heavier frame and braking system too. Just an all around better deal. So, just because you can hook up to something and the tow vehicle of choice might untrack things does NOT mean it will be a good, all around, tow vehicle--especially if it's got a smaller engine and/or highway gears.
Next time you get a chance, watch the Lucy and Desi movie "The Long Trailer". Many amusing and hair raising scenes, which should send a message about tow vehicles. I know, that's all they had back then but the message is still the same today. Of course back then, truck chassis vehicles were not something that some non-agricultural people would be caught dead in.
Most of the Chrysler trailer package cars had 3.23/3.21 gears in them with something like P225/75R-15 tires on heavy duty wheels, plus suspension upgrades and wiring upgrades for the added trailer items. Pretty much like the genuine police package cars of those similar times. I know that people used to two lots of things with their cars, and many still do, but using something similar to a HD 3/4 ton truck chassis with a substantial engine and correct gearing has become the best way to do things in modern times.
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