It's going to take a combination of offset, backspacing, and wheel width to get larger tires. Depending on what width and rim diameter you're looking at will determine the other 2 dimensions. As Bill mentions, tall and narrow 15s are getting hard to get, you almost have to go wider. On the otherhand, tall narrow 16" tires can still be easily found, with 215/85/16 (30.4" tall) and 235/85/16 (31.7" tall) being the common sizes. With the 15s, you almost have to get into floatation sizes to get a tall tire, 30x9.50x15 and 31x10.50x15 being the common ones. The are also a 32 and 33 inch tall 15s, but they start getting rather wide, and really push the limits on fedner clearance, and swell as stress on the front wheel bearings and steering gear. For the 30x9.50 and 31x10.50, you would want an 8" wide wheel, 3"-3.50" backspace, and -25mm offset, which is a standard GM 4x4 fitment for '73-'87 (I'm running this setup on mine). For the 16s, which will the tricky one to find, you would want a 6.5"-7" wide wheel, 3"-3.5" backspace, and 0mm offset, though a little bit of negative offset, say about -15mm, probably won't hurt anything.
