This is one of those gray areas where there is some difference of opinion. Technically the Tahoe/Yukon as well as the Expedition/Navigator are capable of handling the weight of an Overlander, but the question becomes one of how well the combination will react to weather conditions -- gusty sidewinds, passing semis, and sudden unexpected gusts of wind typical of some valleys.
I know that there is a calculator formula discussed in a thread here on the Forums that addresses the ratio between tow vehicle wheelbase and trailer length, but I am not certain its precise operation. My guideline has always been to take the usual gross loaded weight of my coach and multiply that by a factor of 120% to arrive at my minimum trailer tow rating (6,100 * 120% = 7,320 pounds minimum). My current primary tow vehicle, the '99 K2500 Suburban has a trailer tow rating of 10,000 pounds -- I went overboard partly because I like the pulling power of the old-fashioned big block V8 -- it has proven a quite reliable tow vehicle as it now has in excess of 197,000 miles on the odometer. My Cadillac has a trailer tow rating of 6,000 pounds, but due to its hefty big block, has had little problem handling the trailer -- the car by itself weighs nearly as much as the loaded trailer.
My suggestion would be consider the trailer towing capacity you need. Most Overlanders built between 1960
and 1970 will have typical loaded gross weight of between 5,000 pounds (very earyl 1960s) and 6,500 pounds later 1960s through most of the 1970s. Since the Overlander was most often chosen by people towing with large family station wagons or large family sedans the emphasis was on keeping the gross weight within the trailer towing guidelines of those vehicles -- often 6,000 to 7,000 pounds maximum.
Once you have targeted the minimum trailer towing capacity that you want to consider, I would suggest test driving each vehicle that is of interest. You may find that one will stand out as a stellar performer above all others that you try. I know that once I drove a Suburban, I knew it was the only long-term tow vehicle that I wanted to consider.
With the '64 to '68 Overlander, you would likely be safe to estimate a loaded gross weight of close to 6,000 pounds if not a bit more. Tongue weight will likely be around 700 to 750 pounds. For me, the Overlander is more than I would want to tow with the Tahoe/Yukon, but that is more via my preference for the big block V8 rather than capability of the Tahoe/Yukon.
Good luck with your deliberations!