Usually, repeat usually, a small block car will have an 8" rear. Usually, a 351 car will get a 9" rear. AFAIK, all big block cars came with 9" rears.
If your car still has the metal tag attached to the rear end center section bolt, you can read all about your rear. Info will include ratio, size, date of mfg, and other codes with regard to bearing sizes, etc. On the lower of the two lines of info, nearest to the nut will be the rear end ratio. 3.00 for 3.00:1 open. A 3.00 posi would read 3L00. There will then be a couple of spaces, then a single digit. 8 for 8", and 9 for 9". Simple, eh?
If the tag is missing, there are several visual clues to tell them apart. An 8" housing is 'flatter' top to bottom. there is a noticable flat spot on the bottom. Also, if you can use a deepwall socket and a straight extension on the lowest bolts, you have an 8". A 9", with its bigger ring gear, had a bigger 'pig', and gets in the way of easy access to the bottom center section bolts. You almost have to use a box-end wrench on those bolts on a 9"
There have been several different limited slip rear ends from Ford. Back in the 50s and early 60s they used the Equal Loc. Good luck finding parts for these dinosaurs! Then sometime in the mid 60s, the Traction Loc replaced it. Ford also used Detroit Lockers on some of the more powerful muscle cars. The most commonly found posi unit fund on Ford products is the Traction Loc. Since the 9" rear stayed around (in trucks) until the 90s, parts are generally available for them.
The non-locking and loking rears have different ring gear carriers. You'll have to find the posi unit for whatever rear you wish to use. 8" posi units are generally more difficult to find. 9" units are much more prevalent.