I think the numbers on the L3400 are the same as for the L3200/L3800. The R4s diameters are 40.5 and 27. The R1s are 43.2 and 29.1 (for 11.2x24 and 7x16).
The parts manual lists the tooth count for every gear in the tractor. It is a tedious process, but you can come up with the actual gear ratio, rear/front. I did the counts, and got 1.5526. Kubota says 1.56. The R4 tire ratio is 1.50, the R1 ratio is 1.48. Kubota saya they like the front tire to be running about 5-6% fast; either of these ratios work. You want the tire ratio to be just a bit smaller than the gear ratio.
Kubota uses a lot of TITAN tires. Their web site gives a pretty full set of specifications for their tires. Pay some attention to the number of PLYs, and the load rating. You need 6 or 8 ply on the front if you have a FEL. My loader weighs about 1160 lbs empty. Since its center of gravity is in front of the front tires, the front axle is the fulcrum, and you have over a ton on it, again with an empty loader. Toss in the weight of the front half of the tractor, and you are well beyond the rating of the 4 ply tires. Something to think about...
I went for the R4s for the load rating, but I've really regretted the choice. I'd love to make a trade to R1s. But get plenty of plys.