Is an 8 1/4", 2.45:1 diff housing exclusive?
"Units with a 2.45:1 gear ratio have a special differential case that will not accept any other gear ratio." Say it 'aint so!
To answer the question, yes, the 2.45 CASE is a one ratio deal. But AFAIK, the CASE here talked about, is the thing that that ring-gear is bolted to. So then, you should be able to swap in a different case with corresponding different gears. But that gets expensive, when you gotta hire a guy to do it for you, cuz it's gonna require a new set-up; rear gears do not just swap right in.
EDIT;
@motorpirate says it is in fact a no-go, so planB it has to be.
Plan B
The better idea is to swap a complete rear end under there with gears already installed.
And finally, what ratio you install will depend heavily on if the 4-speed that you currently have, is or is not, an overdrive unit, which it sounds to me, that it is. I say that because with a regular 4-speed,and 2.45 rear gears 65mph would be ~2800rpm in 3rd/2020 in fourth, whereas, with an overdrive, third is direct so 65=2020rpm in third/1480 in overdrive.
So no, you cannot climb anything at 1480rpm. I'd be surprised to hear that a slanty could even maintain 65 at 1480, no-lol.
Furthermore
your starter gear with the regular 4-speed and that 2.45 is, 6.52 and that is sure to be very lazy. But the overdrive is nearly as lazy at 7.57, just about 16% better.
So again, which rear gear you choose, will very much depend on which transmission you already have, and how much hiway rpm that you are willing to put up with. And how much more take-off performance you want. You can't really have it all with any of the Mopar boxes, but you can bias them to fit your needs, just a lil.
Here are the ratios;
2.66-1.92-1.40-1.00, the standard box
3.09-1.67-1.00-.73, the od box
3.09-1.92-1.40-1.00, the Commando box.
For a hot-rod get-away, you need a starter gear of about 10/1. To cruise at 65=2400 you need a 2.94 final drive ratio.
The only box that can ballpark that is the od box. As already recommended, the 3.73 Jeep rear will get you a starter of 11.53, and a cruiser of 2.72 so 65=2250, both of which are excellent.
The fly in the ointment is that you only have 3 gears to get you to 65, and they are kindof far apart. So, then, your slanty is gonna need some ballz to get there.
Whereas
with the standard box,and 3.73s, take off is still pretty good with a starter of 9.92, but 65 will be about 3080. You can climb just about anything at 3080, and the six is still pretty smooth up there, but whether or not you want to travel far at that rpm, is a personal matter.
A happy medium is 3.23s and a Commando box for 65=2670, and a starter of 9.98, and now you have four gears to get you to 65mph. It's my favorite combo.
All calculations were done with an 83" rollout which is a tire size of about 26.4 tall. I'm guessing ~205/75-14
Hope that helps
