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.
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.