Gismo, I have no answer for you there. I've got a 12 G with wobbly hubs. I've heard that better hubs can help, but I haven't tried that yet. I drilled and tapped my hubs for two 3/8 bolts, but the bolts keep getting loose. I haven't tried locktite yet, so I don't know if the bolts are working loose or if something else is going on. Right now, my back-up plan is to install the rear end from a JD 210 that was being parted out in my area.
On the 317 I'm working on both wheels were loose when I got it. The large castle nut that holds the brake drum onto the axle was loose. The PO apparently did not realize that there were 2 holes through the axle and you can usually get the axle tight and get a cotter pin through on or the other hole. If not, a shim can be installed so you can get it tight and get the cotter pin lined up. I'm not sure if you are dealing with the same setup. The 300's use a Dana rear end and not a Peerless.
This was discussed a bit in this forum a couple of years ago ("Axle hub fixes - ideas and tricks"). There seemed to be two satisfactory solutions, either drill, tap and insert setscrews, or fill the gaps with a metal epoxy. I went the epoxy route and it seems to be holding up ok.
I used Loctite automotive epoxy putty. Apparently it's best to sandblast the parts but I wirebrushed each axle shaft and hub and then wiped the splines with ethanol. I smeared the putty onto the outside 3/4 of the splined part of the axle (I left the inner quarter because I didn't want to jam up the space between the hub and the axle cover), and onto the inside of the hub, then with the axle vertical,(standing the transaxle on one wheel) I lowered the hub onto the axle and wiped away the bits of putty that were pushed in front as much as I could and all the bits that got pushed back into the well of the hub. Then I found that in the vertical positon, the shaft slipped just a little into the transaxle and I could not get the surclip into its groove, so I heated and bent a screwdriver blade to prise the shaft up a fraction and slipped the clip into place and left it for two days. It probably shifted when I did that, but I can't feel any movement in the hub yet.
I may well need to heat the hubs up if I want to get them off now, but I had completely serviced the transaxle first so I keep my fingers crossed that it won't be necessary.
I'm going to pick some epoxy tomorrow and I will doing the same job on my mf12 axle hubs on the weekend. I will let you know how it came out sometimes next week. Thanks for al the information, greatly appreciated gismo