An 8" wide wheel is about the max width for a 235/75R15 tire. I believe the tire manufacturers recommend a max width of a 7" wide wheel. On my '66 Suburban I am running 275/60R15's on 15 x 8 Rally wheels. Since your truck is lowered, you will likely have to go a little narrower like a 245 or 255. Of course this is also assuming your differential is the original width.
http://www.67-72chevytrucks.com/vboa...2&d=1243225524