At least in 60s terms/era, everything in 1/64 up through a John Deere 4010/4020 should be plausible still in HO. The 5020 was the big one at least when I was down on the farm. Wheels roughly the same diameter as the 4010, but bulkier in appearance and usually with duals, at least here in the Midwest. The 5020 might work despite that, have to give it a look/see to judge if it works for you if you have one available. If lettering is close to prototype, it should be small enough not to call attention to your sleight of hand. 'Cause any farm kid might call you out if he looks cosely
EDIT: Found some pics to illutsrate how even what looks right can start looking too big depending on the situation it's placed in. This is an Ertl John Deere, but no model # is indicated on the tractor. I'm guessing maybe a 3020 or something like that. It doesn't look too bad out on the lot, although it's a really big tractor that looks as big as a semi.
On this DC flat, it's starting to look unrealistically big. While it is converted to HOn3, it started as a ECW standard gauge kit. Ye the width of the rear axle is nearly as wide as this already wide car.
