Yup, with 18x9's you could go 275/35/18 on all 4 corners with a 1/2" offset in the back.
The only problem with that is the offset needed isn't exactly the same in the front as it is in the back if you have an A-body 8 3/4. In the front you need a 25-30mm offset (6" to 6.25" backspace). In the back it's more like a 20mm offset that you need. An 8- 10mm spacer isn't a really big deal, but you will need longer wheel studs.
Moving your springs in just reduces their effectiveness (leverage) in handling side loads (corners). Maybe 1/2" isn't an issue, but moving them into the frame rails seems counter-productive to me. Just a thought.
THIS^^ The leaf springs control car roll in the rear, so ANY amount you move them inwards reduces the leverage they have. It MAY (probably) be that you could make this up with a rear anti-roll bar.
Do the 1/2 inch offset spring kit, I did the 3 inch relocation set up on a car and never liked the handling, it needed really stiff springs to work decent.
Yup, all of this is true. The 3" spring relocation will require higher spring rates and a larger rear sway bar to compensate for the change in leverage on the springs. For 1/2" offset it's not really a big deal.