step side rear tire size rim width

[whatnow123]

Can the step side people help out here? I'm wondering what size tire I can fit in the rear of my truck. I pulled off my bed today because I found the right bed for my truck (came with a 66 step side, didn't know the truck well enough to know when I bought it, but now I've got a 68 bed). Anyway, there seems to be quite a bit of room in there with the bed off.

How wide a tire can I run with a stock short bed?

I'm also now thinking of shortening the rear axle a couple of inches, converting to a 5 lug and "mini tub" the bed. Nothing pro street or anything, just widening the wheel well a couple of inches. Is there a shorter axle that I can swap in there or should I just narrow what I have? Truck 12 bolt, (not sure if I should waste the money on this rear end or not or if there is a more cost effective way?)