That truck is 5x5 pattern. TTII's are easy to come by in that pattern. That's what's on my 68(also 5x5). Summit Racing, Jegs, Newstalgia Wheel, or just about any wheel supplier should be able to get them for you. Also, check Craigslist. Always somebody selling used sets.

9.5" wheels fit good on the rear. 8" fit good for the front until you go any lower than 3" front drop, then they tend to rub, so if you plan to lower it that much or more, then plan to run a 7" wheel up front.