First off I don't think you would benefit from the switch to Dayton style wheels. As far as getting tires swapped if you have a visible removable ring one ore two piece they are not widow makers the widow makers come apart in the middle and will look the same at either bead seat. Yes some shops don't like doing tube type wheels of any kind but you shouldn't have any trouble if you go to a shop that dose a lot of big truck tires as there are still a lot of tube type tires running up and down the highways. I think the problem with putting 9.00 20s on your rims will be that the wheels aren’t wide enough but it is worth checking in to ask at the tire shop you find to do the work. Next if they fit the wheels and clear the fenders they will help with top end speed by rising your effective gear ratio with a taller tire. Last but not least there is a lot of info here in the tech tips on tires and wheels so check it out.
I have a 64 GMC I work the truck so I bit the bullet and switched too 10.00X22.5 tubeless wheels and tires.
How far you go and what you spend will depend entirely on a combination of how you plan to use the truck and what you are willing to spend. I found that with the tube type tires I was having a lot of flat repairs they just seemed as if they were magnets for nails, screws and such. Along with the fact that a simple flat repair will cost you double on a tube type tire and wheel and the down time the switch to tubeless was well worth it to me even with increased usage I went from a flat repair every week or two to not a one in over a year.
