Here's what I got, and the sound is great.
I installed the Kubota radio bracket. Be forewarned - there are very few stereos now that will fit that Kubota bracket. Many of them are now an 1/8" too wide to fit. I found Alpine and Pioneer units (I believe) to be the only ones that were only 7" wide and very limited options with Panasonic now, which is what you will need if you go with the Kubota bracket.
This is the Alpine stereo unit. It does not have a CD player, but is fully Bluetooth compatible and works with my iPhone:
Alpine Mecha-less Bluetooth UTE-42BT Headunit*| Creative Car Audio
There is also a model with a CD player if you want that feature.
I installed these JL Audio speakers. Of all the 4" speakers I looked into, they seemed to have the broadest range, including the lowest lows (bass). I think the key to the best sound is to install the speakers exactly where WaxMan and I did.
Amazon.com: JL AUDIO EVOLUTION C2-400X: Car Electronics
I had to order the Kubota B series cab "speaker covers" to fit in the location where I installed the speakers, since as WaxMan said, the round grilles that come with the speakers are too big. Both the radio bracket and the speaker covers came from the Kubota dealer.
There is a company out of NC that sells the Kubota wiring harness plug for the radio side, so you connect the radio wires to the plug wires, then plug the radio into the factory plug that is in the headliner. Part number is 511655 - $19.99
http://www.agriradiosupply.com/harnesses_&_radio_types_k.html
I noticed that one of the local dealers are also installing Alpine stereos now, since the Panasonics that were originally part of the
B3030 cabs, are no longer being made.