I wheeled the piss out of mine on a Teraflex level lift, longer shocks, and 35s. It wasn't ideal and it rubbed the flares, but didn't see any damage anywhere. With a bit of bumpstop you'll be fine.
If you get some flat flares or trim your stock flares, you'll be fine without new bumpstops, depending on shock length. The only time I had an issue for not having bumpstops on 35s, level lift, and flat flares, was when my wife was driving 45* across a very steep creek and flexed the rear right tire into the bumper. All was well until she quit listening to me and put it in reverse, at which point the tire grabbed the top edge of the rear bumper and buckled it a bit.
34x10.50-15s, level lift, longer shocks, flat flares, stock bumpstops.
35x12.50-15s, level lift, longer shocks, flat flares, stock bumpstops.
All in, I wouldn't recommend 35s on level lift unless you go with flat flares or trim your stockers. You can bumpstop the shit out of it, but then you'll constantly be bouncing off the bumpstops and won't get nearly the articulation you could.