I'm currently running those m/t drag radails 325/50/15 on my 94 camaro z28 on my 15x10 prostars. I'm looking to replace those with drag slicks and I wanted to run the same type of tire size and width What's the widest 28" slick I can run -

 

I run a M/T ET Drag 28x10.5-15W. That has a 13.1" section width, 10.9" tread and 28.1" diameter. Same section width as your 325/50's, and a narrower tread width than the radials. The diameter shown on the M/T site for the ET Drags may be a bit "off", because while my Hoosier QTP 28x11.5-15's fit without cutting anything (did have to move the axle back about 1/4" to eliminate the tread from hitting the metall front edge of the fender opening when the slicks grew over 125 MPH), I had to take at least 1" off the leading edge of the rubber bumper cover with the ET Drags.

Do you have any issue with the 325/50's? M/T shows the diameter as 28.0". You typically do not have "growth" problems with radials, because they don't grow very much, compared to the large growth of the bias ply slicks.

There are photos of my setup in the Drag Wheels & Tires FAQ.