The tires on those 20s in the picture are BFG AT2s and are a 124/127 rated tire. Same tire rating as the 3500. I’m keeping my BFGs and 20s and putting them in my 3500. Stock 20” tires are a 121 rating but the AT2s are 124 so they should work no problem on the 3500.
Ive read expensively on it and shouldn’t be an issue at all so long as the tire is 124 load rating
Note: stock tires are both “E” load range but my understanding is the number that comes after it specifies the tires actual load capacity