Whats the largest size tire I can put on a nonlifted 1978 GMC sierra?

[Blah]

Im going to be trading my car for a nice 78' GMC Sierra! I'm excited that I have a project, I want to know whats the largest tires I can put on it without lifting it,
largest size without rubbing, and then the next size up please.

 

[95gmjimmy]

I ran 32's and 33's no problem on my old one. No rubbing at all so you might be able to go bigger.