any idea on how tall and wide of a tire i would be able to fit on a stock D1 if i put on some 50 or 75mm flexi flares (the ones where you cut out the wheel arches and rear doors a bit) and some 30mm or even 1.5 inch wheel spacers on RRC wheels.

 

You may be putting the cart in front of the horse here.
Have you rebuilt your diffs, new axles and lockers, if not you will be limited by Rovers weak spider gears.

 

not sure what you could fit at stock hight, but people usually run 35 or 37s with a lift and flares. 33s maybe?

 

Without a lift you can't run a 265, only a 245 and they can be snug.

 

those flares open up the wheel wells considerably. 265s would look small. take a look at this link. they try different size tire with a similar type of fender flare.

http://www.eastcoastrover.com/Discotech.html