Do you guys think a set of 30x10.50x15's would fit under my red truck below. 88 C1500, silverado.

If not will a set of 255/70/r15?

I would have to say "no". But don't take my word for it, i'm just replying because no one else is.

I'm pretty sure my brother has 30x10.5r15's on his 98 c1500 2wd.

I will say yes, even though I can't see where this red truck is

it's the bottom one ( I rattle canned the top and bottom black (to cover up the dings and scratches and stuff) so it would have a two tone look). I forgot I only had a straight forward shot of the lights. It is the same as my green one....just older.

If your pickup gets mostly street abuse, I would suggest a set of 235/70-15 all around tires, like some Firestone Firehawk tires. They will help in highway handling and traction.