What size tires is everyone using?

I have a 90 c3500 with a cummins swap in it and the tires that are on my 16" alcoas are no good and cant really handle the weight of the new engine. I was looking for some advise. The truck is lowered and I do like the look of it but I need some tires so I can drive it around.

Here is a picture of my truck. What sizes are people running and what is the load range of them