The 1976 Overlander has an empty weight of either 4,575 or 4,600 pounds with an empty hitch weight of between 500 and 550 pounds. Realistically, with options and personal belongings added the gross weight will likely approach 6,000 pounds with a laden hitch weight of 750 to 775 pounds. My '64 Overlander has very similar empty figures with a gross weight of 6,100 pounds and a laden hitch weight of 750 pounds.
I have towed the trailer with both automobiles and light trucks. While a small block V8 with the correct gearing CAN handle the chore, a big block V8 will be much better suited to the load. I currently tow with either a '99 K2500 Suburban (7400 VORTEC with 4.10 gears) or a '75 Cadillac Eldorado Convertible (500 cubic inch V8 and 2.70 gears). Each vehicle has its benefits, but the Suburban handles any kind of hilly terrain with much greater ease due to its gearing. My previous tow vehicle (before the Suburban) was a '95 K1500 Z-71 Extended Cab Pickup (5.7 Liter V8 with 3.73 gears), and it was not able to adequately tow the weight in the Rocky Mountains - - it was in Low gear at less than 20 MPH when we cleared the Eisenhower Tunnel on I-70.
The minimum tailer tow rating that I suggest looking for to tow an Overlander is one of at least 7,500 or better yet 8,000 pounds. My Suburban as ordered, came equipped with a 10,000 pound trailer tow rating while the Cadillac as ordered came equipped with a 6,000 pound trailer tow rating (one reason the Cadillac sees minimum usage with the Airstream is that I must seriously adjust my packing to keep the weight below its maximum rating).
Good luck with your search for an appropriate tow vehicle.
P.S.: The following link will take you to the factory site with the Airstream trailer weights as well as other valuable information.
Airstream Weights and Measures