Wolfman,Yes the ratio is the most important thing to maintain, generally 4% deviation max is a good rule.
The real issue he is having is trying to stay in the same rim size.
I've been taught by the best engineers in the industry that 2% is the golden rule you have to play with when it comes to the tire size/ratio differences.
And in figuring it this way I've never encountered a problem whether it was a B7100 Kubota or Case/IH8930 zero problems when the changes were done.
If you know something different please enlighten me of the particulars as I do this quite often making a living selling tires.
Al