I don't know the dimensions, it would be easier to transfer the holes from the engine. And keep in mind, adapter plates are an exacting science. It must be extremely accurate to get the proper input shaft alignment. Any error and there will be binding, bearing failure, and transmission damage!
Something else to consider, what are you going to attach the front suspension to?
Do you have a plan for a governor?
Room for the carb or fuel system under the tank?
Automotive engines have an entirely different power band than tractor engines. That tractor is geared to work between 500-2000 RPM. Unless you change the cam, the V6 will not be on the power band unless you have other plans than actually using it as a tractor.
Not questioning your ability, just don't know how deep you intend this project to get. It is not an easy task by any means!