I have a 2018 Savana 2500 8-lug with 245/75R16 tires.

I also have a set of chrome wheels from a Ford F-350 King Ranch 8-lug with 275/70R18

Both trucks use a 8-6.5" bolt pattern - but . . . . will they clear the fenders? <g>

 

This is a good tire size calculator.
https://tiresize.com/calculator/

You'll have to see how much clearance you have and how much you need to clear if you hit a bump with the van already loaded down and squatted. You might try loading a bunch of weight (people are pretty easy) in the van and jacking up one side of the axle till it lifts the tire and the raises the van. Give yourself a bit more clearance that that to clear hard bumps.

Another thing to consider is the offset of the stock wheels vs the offset of the Ford wheels. If the offset is different then the wheel will ride farther inside or outside of the wheel well.

 

I think I've got the biggest that will fit. They rub the top of the inner fender when you hit a big bump (only ever had this happen on bad dirt roads), and rub the anti-roll bar at full lock. LT265/70R17 on Dodge 17x8 wheels with 41 or 43mm of offset if I really correctly. I had to bend and relocate the front wheel speed sensor wires and bend the parking brake bracket on one side of the rear. You can't go wider or taller without more modifications.

 

From what I found online the F350 18" wheels came with either a 40 or 44mm offset which will be similar to the Dodge wheels used by Triaged.

The 2018 Savana 2500 wheel offset is around 29mm so the Ford wheels will ride about 11 to 15mm farther inside the well on a 2500 van.

I've been entertaining the idea of swapping wheels lately too so this stuff is right at my fingertips.

 

On average they sit further inboard. The outside is in about the same spot as factory (maybe out slightly) with all the extra width going inside. I was able to finally find hub-centric adapter rings for the Dodge wheels. I've posted about both the wheels/tires and rings.

 

Someone else also has info on the Dodge wheels. I wonder if an 18" wheel would clear the wheel speed sensor.