measure around as close as u can to the outside wear holes to get your stagger..that should be close to what your looking for...and when the tires are on the kart on the track the tire patch will increase...
Have you tried to shrink them? Normally it just looks like it has a slight dip. After shrinking one down to 34 inches it will have a dip in the center. The old 800s had the dip when you got them to a useable size. The newer style 800s, right before the 900 was the only thing they sold, I haven't seen a dip. Have 2 sets of newer style and both are shaped and size out of the box like a 900. Have 4 sets of the old ones and they all have dips in the center and rollout at 34 1/4-34 3/8.
Done it different ways. Some have a spare junk oven they use. I use a hot box most of the time.
Set it to 105-115. Take valve core out put them in the box if they are mounted and roll them until you can barehand grab them but dont want to hold onto the tire. Then i put it in the freezer to shock it. You can get a bucket of cold water and when you take them out of the box hot pop valve core back in and dunk tire in water until its cool. Shocking the tire will drastically help to hold stagger. If i dont shock tires in freezer or water stagger will change at the track after running them. You will probably have to shrink an old 800 2 times before it comes down. They are stubborn. Before mounting them heat em up and shock them. Repeat after mounting if necessary.
If your box won't roll a tire without air or unmounted from rollers being to far apart. I have used a torch or heat gun to shrink a tire. Heat gun preferably, same process get them hot enough you dont want to hold onto it and shock it.
The new style 800s and 900s are fairly easy to get to change so expect to have to swell it slightly after shrinking to get the exact size you need.
If you want to stretch a tire I hold stagger tape around tire. Air up to the size I want it. Heat it up, then when you pull it out air it up to 1/4 bigger than where you want it and shock it cold. That should get you close. The 900s I've aired up where I want heated them and had to let a little air out to get them 1/8th to 1/4 bigger than what I want. 1/4 will get you close on lefts and rights. May need to adjust depending on how hot you get tire. When it cools let air down to 5 to 7 pounds and let sit for 5 or 10 minutes then check size. They will change about an 1/8th inch until they settle.