can I put a 10" rim and a 315/35 w/o modifying the sheetmetal?

No.

CrossRam is right . NO!

Only way it might work is if you had airshocks and 3.5"bs wheels, but then they'd stick way out of the wheel wells.

I have 285's and they will rub on a hard turn or off camber turn like into a driveway. Also no way would I put weight in the back.

I had to run my old L-60/14's on 9" Fenton slotted mag's with 3.5" b/s using coil-overs that raised my rear up about 4"' or so.

They measure a MODERN 298-53/14!!

Still got the big suckers mounted up on my top shelf!

pdq67