Seriously, the rear isn't an off-the-shelf measurement now that it has a Ford 9inch.
For the front, a 20x8 will work nicely with 4.5" backspace, or 20x8.5" with a 5" backspace.
Again, the rear you will need to measure for proper backspace, unless the replacement Ford 9inch is the same exact width (drum-to-drum) as the OE GM 10bolt.
As for tires, it depends how big you want to go, how much you have cut and whether you want to lay frame.
255/35/20 front and 285/30/20 rear
245/40/20 front and 275/35/20 rear
255/45/20 front and 295/40/20 rear
245/40/20 front and 295/40/20 rear
255/50/20 front and 305/40/20 rear
265/50/20 front and 295/45/20 rear
265/50/20 front and 275/55/20 rear*hehe you might see this on my gmc............... maybe.