I believe 300G would have had 8.00-15 tires (29.3 to 29.7 in OD, Static Loaded Radius 13.5 to 13.8 in - 1960 Tire co OE tire data) - variation by manufacturer - larger is General, smaller are BFGoodrich and Firestone - sorry no Goodyear data but should be close to BFGoodrich / Firestone.
Only 6 common 15 inch sizes that vintage (most cars went to 14
inch in 1957, but these were previous generation and some 15 inch
holdouts) - 6.40-15 Rambler, Studebaker; 6.70-15 Chevy, Ford,
Plymouth; 7.10-15 Pontiac, Mercury, Dodge; 7.60-15 Buick, Olds,
Chrysler, larger Pontiac / Mercury wagons; 8.00-15 Lincoln,
Imperial, bigger Olds, 300B & 300G; 8.20-15 Cadillac. Some
limos 8.90-15 but I never saw one.
So the 710R15 are definitely too small for a big 300G - 800R15
would be correct if it still exists (Check American Classics or
Diamondback Auburn Premium) and you might get away with 760R15 if
such exists but still small to replace 8.00-15.
In current sizes P235/75R15 (28.9 in OD typical, 12.9 in Static
Loaded Radius typical) is about as big as you will find - load
wise they should be OK, but they will sit lower. Check Auburn
Premium (3 WW width options) or Coker American Classic.
Maybe Maxxis MA1 and Gremax 5000 too but I cant remember if those
are wide or narrow WW. Lots of high quality production tires still
available in blackwall P235/75R15 (under $100) if you want to go
that way and have Diamondback put custom whitewall on them - (net
price about same as American Classic or Auburn Premium?).
As to using tubes, that was not an uncommon problem with early
wheels when tubeless tires came out in 1955 and it was
particularly true for some wire wheels even into the early 70's.
You can always put a tube in a tubeless tire. Just make sure you
have the correct size tube (never go oversize - they will pinch
themselves) and one rated for Radial tires (some tubes are not
rated for Radial tires). And if your wheel has protruding spoke
ends, make sure there is a rubber sleeve covering spoke ends to
protect tube.
Tires with tubes will run a bit hotter and if you pick up a nail
they will go flat where a tubeless tire will maintain pressure
much longer if the nail remains in the tire.
Best, Ed
