few question

i have a 98 tahoe wanna do some modifications
what do i have to do to fit 32x12.5 tires on my truck is it a simple body lift if so how high also could i use addaleaf and crank the front torsion bars to get the desired height also right now i am thinkin of gett in 31x10.50 tires are they any taller than the stock 265s that are on there

another question i went to a auto parts store to look at posibly adding a flowmaster and through one company i found that the 70 series would fit but the auto parts store told me i would have to do a cat back system because the exaust is designed differently than most truck

thanks