The numbers you have are the casting for the case. Your O8AR 7008-D would actually be C8AR 7008-D. That is a case used for a 68-71 vehicle. Unfortunately, that is all that we can tell from the casting #.
The toploader cases are very universal and the same case is used for big blocks and small blocks. Also for close and wide ratio transmissions.
There should be a tag riveted to the passenger's side of the case that will have (in the above casting) a "RUG" ID. Unfortunately since these cases are universal plus being 30 years old, it could now be built in any configuration. Happens a lot and I am guilty of it too. Here is a picture of a similar case.
The tag can be seen on the LH side of the pic. Normally it is not covered in primer.
A Cleveland will use a small block toploader and you can use either a close or wide ratio.
Have the owner measure the diameter of the splined area of the input shaft. If it is 1 1/16" in diameter, then it is possibly for a small block. It could also be for a 390 since it also received the small diameter input shaft. A 427, 428 big block has a 1 3/8" diameter shaft.
If the tranny has the small input shaft ask for the length of the tip of the input shaft. If it is 1 1/4" long then you have a small block tranny. If it is only 3/4" long, then it is a 390 tranny. This shaft is not directly usable behind a small block.
Here is a cheat sheet that I drew up long ago:
The only other problem would be the tailshaft and the tailshaft housing. This is easily measured. Big cars, such as a Galaxie used a long 17" tailshaft housing. Mustangs used a 14" tailhousing. You will want to use the correct one for your vehicle as shifter location is important as is the drive shaft length. If the gentleman can give you the casting # of the tailhousing, then we can give you a better idea of what type of vehicle it came from.
Hopefully this brief intro should give you enough to get started.
My little 65' Stang street car:
Dart 428W NA, 4 Spd, 4:33, on pump gas pushing 3550lbs.
firstname.lastname@example.org with a 1.466 60'