I just bought a 65' coupe. it has a 90's model 5.0 and they also put a 5 speed in it. it's pretty strong but i think it has the original rear end. Is there an easy way to tell by looking?

 

if the chunk will come out from the front it is probably the original 8 inch. if it has a plate on the rear that exposes the gears, it is probably the newer 8.8 inch rear end.

 

Like he just said, look and see if the rear end has a removable cover on the back or not. If it does it is probably a late model 8.8. If it doesn't it is most likely an 8 inch, but could also be a 9 inch.

If you have a tach, What RPM are you turning on the highway in 5th doing about 70?

I would recommend 3.50-3.70 gears.

 

If the diff is a removable piece, with studs and nuts around the front, it's a 8" or 9". You can use a socket and 6" extension on the nut of the 8", but the 2 lower-right nuts on the 9" would be blocked by the pinion housing.

The 8" will be fine, if you are not racing the car. The 8&9 were identical in design, the 9" was just 1" larger and beefier. Currie has a 4-pinion Traction-Loc diff if you need to convert to limited slip, this is stronger than a lot of stock 9" diffs.

 

it doesn't have the bolt on removable plate, so it is probably the original (it looks 44 years old too) what are the lowest gears i can put in the original 8 inch? will 3:70's fit?

 

You should be able to run 3.70 gears in the 8 inch. I would upgrade to traction lock and replace all the bearings and seals while its opened up.