Overdrive Manual Upgrade
From Mopedia
Unless you are going to eliminate the torsion bars, every choice you have listed is a non-starter except the A833. This unique feature of classic Mopars makes it difficult to install a physically larger transmission. Unlike cars that use coil springs, Mopars support suspension loads low in the body via the torsion bar cross member. The transmission must pass below this structural member with the transmission mount sandwiching the tail shaft between itself and the torsion bar cross member. If a transmission is too large it will hang down too low and make drivetrain alignment impossible. Altering the torsion bar cross member needs to be done with care as it provides a large portion of the unitbody's structural integrity.
Ther T56 is bigger and weaker than the TKO that Keisler sells. I spoke to Keisler about their 6 speed kit and discovered it requires alteration of the torsion bar cross member. I assume the Tremics in the new Vipers are bigger than the T56 but that is an assumption. This basic gear set is used in many transmissions and transaxels so maybe a custom case that fits would be a possible aftermarket project.
The NV4500 and NV5600 are poor choices because in addition to their large size, they are heavy, slow shifting, and geared for heavy trucks. Unless you are going to mount your car on a truck frame, forget about them.
The NV3500 might be a possible choice. I don't know how big it is but it looks comparable to a TKO at first glance. It has marginal strength but a 318 in a Satelite should be within it's design parameters. It is still geared for a truck and shifts slow so it would probably not be much fun after all that work.
The A833 has a lot of overdrive options. The factory offered a version that used an undersized third gear to provide overdrive. The 3-4 shift lever was flipped upside down to put the overdrive third gear in the lower right position of the shift pattern. The OD A833 came in two lengths just like the non-OD version. The short tailshaft was used in A and F bodies. The long tail shaft version was used in trucks and vans and has all of the shifter mounting provisions for B and E body cars. This design had a couple weaknesses. The overdrive being in 3rd meant a lot of wear from cruising in third. The transmission came with an aluminum case that was actually pretty strong but had a floating counter shaft. It is common to install bushings at rebuild time. This isn't the strongest transmission ever made but there are people running them behind big and small blocks and they seem to live OK. The wide gear spacing is a problem if you are dealing with a narrow power band. The biggest problem for a swap is the huge front bearing retainer on the OD trans that will require machining most belhousings if it is not changed. You simply use one off of a big block. There were basically 4 different bearing retainers. Small pattern small diameter, small pattern big diameter, Big pattern big diameter and the o/d one is big pattern super big diameter. So bolt on the big pattern and big diameter and it will simply fit most belhousings. Ma mopar felt they needed to make the o/d one bigger because it was aluminum.
The aftermarket has provided some overdrive alternativess to Ronnie Sox's favorite four speed. Passon Performance has released an 18 spline overdrive gear set that is claimed to be as strong as any hemi gear set. They also offer aluminum cases and tailshafts. The tail shafts are important because the factory never offered aluminum tail shafts and the Direct Connection ones are impossible to find. Gear Vendors offers their planetary overdrive for the A833 as a tailshaft replacement. I think it is also aluminum. The OD unit sits back from the transmission so it can fit under the floor pan without alteration. The prop shaft has to be shortened significantly which is a drawback as it makes drive train alignment more difficult. This is an expensive unit but it does provide an 8 speed.
