First off, I'd go to another store that has more sense then these people.

But, if you must deal with this shop, I would CALMLY go back into the store and and tell them politely that you know exactly what you are doing, and you are prepared to deal with any rubbing issues and things like that. If that doesnt work, get loud. Tell them you know what you want, if not you will go elsewhere. Hell, print out a picture of one of the many 3rd gen 4Runners with 285's and show him.

88 4R, 350 V8, R150 5 speed Sold <...uot; />
97 4R, stock, daily driver
98 Sienna, kid and dog hauler, wife's ride