I have similar dimensions and I have 2 4"x4" 12v CPU fans, one in take, one exaust. . and a mini clip for cirulation.. seems to work real well for me. . .My cab is sealed real well, and might have something to do with it. . I silconed all none moving joints on my cab and used rubber weather strip tubing for the doors, takes care of light leaks and a nice control of air circulation.