35 vs 37 on 18's

So my few questions are.
What is the difference between these being able to run 35's or stepping up to 37's
I'm currently on 33x12.50R20's i'm liking the tires I have but want to drop back down to 18's and either 35's or 37's the brand of tire only offers up to 35's I believe. I would be getting a oem steel 18" rim set.
Im on the Toytec Ultimate 3" lift (no AF.M UCA's)

1.) would 35's or 37's have issue with the stock steelie back spacing?
2.) would I be broken hearted that I chose 35's and not 37's
3.) is there any thing different thats needed when stepping up to 37's

any input , suggestions, pictures is welcome.

 

Oh and a side note the 35's I'm looking at "True height" is 34.8"

 

you will probably need a better offset than stock steel imo. something in the +18/+25
35's is a nice size for Tundra but stepping into 37's territory and you are opening a can of worms.: much more stress on suspension/steering components/body cutting/ re-gearing/ new UCAs..etc..
They do look good though.
Or you could go with a smaller size 37's like K02s vs Toyo.

 

I believe mountun goat has 37s on stock steel wheels.