I didn't fly for a Grand Canyon tour but a friend of mine did before he was picked up by Continental Express.
He flew at King Aire, starting in C206/207s and eventually upgrading to the twin cessnas. I know he flew his pants off, especially in the summer easily 100 hours/month. Once he started flying twins, he logged a ton of multi in a short period of time and was a great candidate for Continental Express. But he said the only draw back was his IFR skills went south because they rarely flew in the clouds and even when they could, they could only do so for a short amount of time. And most of the clouds were thunderstorm related which they were constantly dodging.
I'm not familiar with 135 regs but there are some funky regs(which maybe someone could expand on). When he was prepping for interviews, he bought a ton of frasca sim. time to get his IFR skills back up to par.
I would do a search of every tour operator in the Las Vegas area and contact each one about time requirements and so forth. Some required the 135 VFR mins and some 135 IFR. It seems like a very quick way to build the time, but a drawback could be letting your instrument skills go south. good luck
