Tech article by Jon Hanna (Admin)
Automatic Transmission Intro
See also Manual Transmission Introduction

C6 3 speed automatic
1968-1979 Ford 4x4 trucks. You'll need the 4x4 tail housing and 31 spline output shaft. Also be sure the donor vehicle has the same engine family you plan on using. (ie. Small block (289/302/351C/351W), big block (351M/400/429/460), or FE (331/352/360/390/391/427/428) )
The C6 case and bellhousing are a one-piece design. Also see the transmission pan image above for identification.
Adapter required?
A tail housing adapter is required as well as a new spud shaft.
The C6 was originally put behind some of Fords most serious big block engines. It's known as a practically bulletproof unit that can certainly handle any small block, especially if it's built up a little. If you're looking at swapping in a big block engine the C6 is one of the few choices because you can find one with the correct bell housing bolt pattern.
Expense, availability and size are the drawbacks to the C6. The small block C6 is fairly rare and may be costly. The C6 is quite a bit heavier than the C4 and hangs down causing possible clearance problems with the front drive shaft. The larger C6 also requires more horsepower to turn and has the same gearing limitations as the C4.
Cost: (Approximate)
Transmission: $200-500 used and $800-1600 rebuilt
Adapter kit: $650-700
Drive Shaft Mods:
Total transmission length with adapter is approximately 2.5" longer requiring some drive shaft modifications.
Bell housing:
The C6 came behind small and big block engines. Because the bellhousing is part of the transmission you must get your C6 out of a vehicle with the same family engine you plan on using in your Bronco.
Modifications needed:
An aftermarket transmission shifter or modification to your stock automatic shifter will be required. The front drive shaft may be very close to the transmission pan depending on your lift. A smaller diameter driveshaft may be required.

Manual Transmission Gear Ratios1st2nd3rd4th5th
Bronco 3 spd (6 cyl)3.411.861.00
Bronco 3 spd (8 cyl)2.991.751.00
NP 4356.693.341.791.00
Toploader (Close ratio)2.321.691.291.00
Toploader (Wide ratio)2.781.931.351.00
Toploader (Overdrive)3.291.841.000.81
NV4500 (Dodge)5.613.041.671.000.73

Automatic Transmission Gear Ratios1st2nd3rd4th5th
Bronco C42.461.461.00
AOD w/ low gear set2.841.551.00.67