Its a good stretch but not ridiculous. The 40 wall helps it. If you decide to go thinner with a 35 series your still safe.

is that a big stretch or just a light stretch?

215/40/18.

What's a good size tire to stretch on a 18x9 +12 offset they are te37 knock offs

North in Fort Collins, with a deposit I would be willing to deliver. The car is what it is, perfect except the very minor bads listed.

you didnt live so far man
what part of CO you live in?