I'm not sure their will be any difference as the 34 are 75 profile while the 30 are 85 profile. My new m7060 hsc12 cab the rears are very close the fenders, can't fit my fist between them. You should look up each tire on the manufacturer page to get complete story and read reviews
There won't be any height difference. Have to keep the same ratio front/rear for the 4WD to work properly.
If you have a choice, and you can afford the cost, get cast centers. You'll need the weight. Might consider having the rears filled in the purchase too. Kubota tractors have a light weight/HP ratio.