Turf tires will work well enough with weight.
I made 6" extensions out of 8" dia stock. Filled the wide tires with fluid at 5-6 psi.
That Gravely 812 would waddle up most any hill.
Let me know if ya ness extensions on the Furd.
And there is a tractor graveyard near me too. Has some lil 4X4s too.
Thanks Rob !! Got to talk him out of it first,hes had it 2-3 yrs. and done nothing with it yet ! Bout time I did ! LOL