Steel Rally Rims & Tires -- Max and Proper Sizing

[1986swb]

I'm new to this forum and I'm sure someone has asked/answered a similar question regarding rally rims and tire sizing but I appreciate your experience and insights.

I have a 1986 GMC Sierra Classic SWB, 2WD. I currently have the stock 15" x 8" steel rally rims on the truck with truck tires: 235 x 75. The truck has a stock 305 H engine, rebuilt 700R tranny, and is bone stock in ride height and in features (full power everything, including cold AC).

I would like to change all 4 tires to passenger tires of either 255 or 275, 60series, but I don't know if 1) the height gap between the fender and tire will look too wierd (large like a 4WD) and 2) if I decide on the 275 sizing, will it fit neatly under the front fender wells.

Any insights is greatly appreciated