What is the biggest tire I can fit under completely stock 2010 f150 stx super crew 18 inch rims I have set of fierce mt 35 x 12.50 what would I have to do to run them and I can trim the front

 

Killer run-on sentence, man!

And, you didn't say if your truck is 2wd or 4wd. Either way, even if it's 4wd, I don't think they'll fit on a stock height truck. You'd need a 2-2.5" level on a 4wd truck. And even then, you may still have some rubbing.

A 33" tire is probably the most you can fit without a level. Again, that depends if you're 4wd.