mini tub

The length of your axle will determine backspace. Asking backspace and thinking you can put it in with a stock width A body rear is going to result in major disappointment. Try putting a 3.5 BS 10" wheel on a std width A body rear... NO CHANCE it fits. It will need around 7.5" bs.

You'll have about 14.5 inch of well to work with and unless you trim the front well edge or move the rear back some, you are around 28" max diameter.

Get your wheel/tire combo, put it in the well and measure your total axle width needed and cut it to that width.