My 73, Pinto wagon does the same thing. Right around 70 MPH and most noticeable when I let my foot off the gas. I have not been able to figure out why. I do not have power steering and '73's have an odd one year only steering rack. I've checked my wheel bearings and ball joints but found nothing obvious.

The ATF might be a leaking steering rack..., or the transmission cooler lines as they run to the radiator. I would clean the area thoroughly, tie some white cloth strips to where you think it might be leaking and keep a watchful eye on them.  That might help to pinpoint where the leak is coming from.

I don't know if the V6 had the C4 transmission. It is desirable over the C3. I don't know if it is the situation here but a lot of time transmissions can be identified by the number of bolts on the pan. An internet search might provide the identification needed.

13" tires are not likely going to be found at the tire store down the street. Places can order them..., if that want to be bothered. There are on line sellers like Tire Rack. They will ship the tires to you, or a participating store for installation (side note the installing tire store will ALWAYs tell you they would have beaten Tire Racks prices..., but they never advertise that).  At the Tire Rack site you have the option of selecting tires by application or specific size. There are other on line sellers but Tire Rack has prices that are competitive. I'm outside Los Angeles and Tire Rack ships out of northern Nevada. Often I have my tires within 48 hours. Sometimes a bit longer.

Anyway, hope you get everything worked out and enjoy the Bobcat.

correction from my previous post:
In 2006, we replaced all 4 tires (Firestone) with P165/80R13, white walls

In the search for 13" tires, the internet is your best friend.  I purchased five whitewall 13" tires for my '73 Pinto Runabout a few years back from a seller on eBay.  If I remember correctly, they were shipped out of Canada. 

Dwayne