Would I be able to throw 305s in the rear on a 10.5" wheel with +20 offset? No suspension mods done.

 

That's a 100% no if completely stock. Even with camber dialed to the max, I would imagine the tires will still rub.

 

+45 offset would get you there, maybe a +40.

 

In that case, what's the widest tire I can put with that offset without dialing lots of negative camber?

 

With a +45, would the 305 tires tuck in or out?

 

Have you looked in the Wheels and Tire forum? There's a thread devoted to questions like yours.

 

Yes I have. Everyone seems to be dropped and running some negative camber

 

Here's a thread from VossenG37S, he's run a lot of combos and this is a 305 on a 10.5 +45, but yet another case of all photobucket pics being vaporized
But his camber is at -1.7. I remember those pics and it was a nice fit FWIW.
https://www.myg37.com/forums/picture...and-clean.html

 

Damn I wish I could see the pics for a visual of what it would look like..was he dropped on that setup?

 

All his setups are on lowered suspensions, and Photobucket deserves to die a painful death