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I am at a tipping point with my car. I have spent quite a bit of time undoing the sins of it’s past. It has a new zz454 crate motor, and a rebuilt Muncie M20 4 speed. Working front to back, the next step is the rear end. It has a 10 bolt 355 with an open diff....horrible. I am ready to swap the rear end. The horsepower and the torque are good...I just don’t feel like I am getting the power to the ground. I am thinking about a Mosier 9” 373. I am just looking for some advice as to the best way to make the car faster. Would my motor/trans/rear end setup above make a significant difference? From what I have read, the combination working together means everything. I am not prepared to do something different with the tranny, but I feel like it’s workable. The suspension is basically stock. Any input would be much appreciated. Understand I am a bit of a newbie when it comes to A lot of this.
 

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M20 is a wide ratio transmission, which means 1st gear is geared lower than the close ratio transmissions. How do you use your car? Is this just a street cruiser, or are you racing it? 3.55 gears is plenty low for a street driven wide ratio gear box IMHO. I'm running a close ratio transmission with 3.31 gears on the street, and they are about perfect for cruising around and occasionally stomping on it. A 10 bolt rear with work just fine as long as you aren't getting tons of traction. With a BB, though, I'd want a sturdier rear axle if the car is hooking good. I guess the first thing we need to know is how you use your car, then an educated recommendation as to what gears/rear end you should be looking for can be made.
 

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Going posi will make a difference. With the 454 I would stay with the 3.55 or go 3.31. 3.73 will not be very good at freeway speeds of 65-70 mph.
 

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Echo these guys. I have 4.11s with my small block with 4-speed and they are too low for me. With the grunt of a 454, it will pull the taller gear well and still have reasonable RPMs at highway speeds. That matters is this is primarly a street car, if you want straight line acceleration, the 3.73s will definitely help IF you can hook up. The 454 with 3.73s might just turn it all into tire smoke.

The Ford 9" will definitely help in the strength department and a TruTrac should ensure both wheels bite. I was told but a local shop you can't get a TruTrac for the origianl 8.2" 10 bolt but it is available for the 8.5" 10 bolt. I only mention that because someone put the later model 10 bolt in mine so don't assume yours is stock. If you do happen to have the 8.5" 10 bold, it can be built pretty strong, some say almost as strong as the 12 bolt. So, if it is not a car that is constantly getting hammered on the track, you might save a few bucks (a few thousand?) if you get luck and have the 8.5". If you have the 8.2", I'd probably want to replace it as you have planned. I may have taken me 50+ years to figure it out, but I know I can't be trusted to keep my foot out that 454!
 

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As said you would be good with 3.31 or 3.55. I run 3.73 with my SB and it pretty limits where I can go. Never hit the freeway anyway. 3.73 you would be at 3000 rpm at 60 mph with a approx 26" tire.
 

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I'm collecting the parts needed to change my mono leaf rear to a '72 Nova 8.5" 10-bolt with multi-leafs. I've got the donor rear, Hotchkis 1-1/2" leaf kit, 3.42 new ring & pinion, Right Stuff rear disc kit, sub-frame connectors. So next to get is a Yukon POSI set up for 30-spline axles, the axles themselves, all the bearing stuff, and once the rear is aligned, have to cut/balance the driveshaft. Its just a driver/cruiser and the Super T-10 will be fine for me. The last thing will be to weld in the sub-frame connectors.
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I'm running a 3:55 behind a 454 with a M20 and find myself begging for that next gear at highway speeds.. IMO, I would not go any higher than a 3:55 unless you go to an OD trans or Gear Vendors Overdrive
 
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