The diecast model is probably a 6480 or 7480? Either way that is a 6 cylinder tractor. The 6400 serie has been produced in roughly three scales; small 4 cyl frame, medium 6 cyl frame and large 6 cyl frame.
It depends on how realistic you want it to be, but ideally you need to sqeeze in a 6 cyl engine.
I dont have the measurments of your build but looking at your build, the overall size and wheelbase and size of the cab compared with the rest, I think a 6465/80 or 7465/80 comes closest to your build, so the mdeium 6 cyl frame. In this case the wheelbase may be a little too long but the overall shape is very recognisable!
The wide fenders are spot on! They look really good.
Im not sure how wide the 45mm wide lego tractor tires would be IRL, but I know that IRL it will take 710 mm wide tires in order to stick out a few centimeters from under the fenders. Our 6480 had 650 mm wide tires and being on the widest track with, both wheel and fender were evenly wide. I believe the 6499 from Farmer Phil has 710s. But, as mentioned before, the 6499 is a large frame and your diecast probably a medium frame. Although visual differences on photos etc. might seem little, a 6499 is considerably bigger than a 6480, while the cab remains the same size.
Obviously it is up to you, but I would go for a 64/7480.