Personally, I would leave the '36 rearend in the car, they are good rearends. I'm not a diehard restorer but I was raised by one. I think a flathead powered car should have early running gear. Install some Lincoln brakes if you want it to stop better.
If you still want to change the rearend Frank Oddo wrote an article on rearends for street rods in the May 1991 issue of Street Rodder. Here is the width chart from the article.
They make brackets that weld to the housing so you can use '36 Ford rear radius rods to install the rearend and still keep your buggy spring. American Rodder did an article on installing '36 radius rods on a Ford 8" rearend in the February, 1998 issue. If you want a modern rearend this is the best compromise, much better looking than coil springs or parallel leaf springs. You definitely do not need a Ford 9-inch rearend you don’t want the extra weight. When I was kid I did horrible terrible things to Ford 8-inch rearends and never broke one. In the article they cut there own mounting plates and welded them to the housing. I believe Specialty Ford Parts in Rosemead, CA has these brackets for sale (626-288-3311) You will need straighten the housing if you weld these brackets on it.
Most of the Ford 8-inch rearends came high gear ratios 2.80:1, 3.00:1, 3.25:1, these are two high for your little 21 stud flathead. Your car will not have the get up and go that it did with the stock gears. Something to think about if you have to buy lower gears.
I would save the old rearend so you can always put it back if you change your mind or decide to sell the car. Don't do anything that cannot be undone.
Jerry