I'm seriously considering using a JVX adapter kit and putting a four speed automatic behind a stock 375 HP 440. It looks like a 42RH or 46RH would be suitable. I know nothing about the torque capabilities of either unit. Would the 42RH be adequate or would the 46RH be a better choice? Again, keep in mind my experience stopped with the late 70's Torqueflite so this is all uncharted territory.

The 46RH were based on the 727 Torqueflites and I believe the 42s are based on the lighter duty 904s. They didn't put 904s behind Big Blocks for a reason.

I used a 46RH in the 37 Dodge PU that I just got running. I'm impress enough with it I'm considering building another one to replace the 4 speed in my 57 Plymouth.

Is the 46 and 42 the evolution of the 518 and 500 ?

A9xx evolved in to 42/44RH then RE.
A727trans evolved in to 46/47/48 RH/RE.

The numeric coding was adapted by most of the OE's to show number of speeds and relative torque capacity.

A500/518/618 are useless designations when you compare to the industry example above.

46 = 4/number of speeds and 6/relative torque capacity.
47 has same number of speeds as the 46 but more torque capacity, 48 same number of speeds but more torque capacity.

As for what to use, you don't mention what vehicle you are going to install the unit in. A passenger car used for normal driving would have no issues with a 42/44. It offers a lower first and second gear than the larger units, is lighter too. All of them have lockup.
Consider that the 44 was used in Jeeps and light truck, so if the unit is built correctly you should have no issues.

If you are going to flog the unit more than drive it you may want to consider the large box.

(Reply to FrankenDuster)

No evolution, they are the same thing.
The naming the 42 or 46 is usually a bit clearer which type of transmission is spoken off.

46RH = H stands for Hydraulic, can be either with just OD, or OD+Lockup. Resp. 2 or 3 pin connector.
46RE = E stands for Electric, and has a round multi-pin connector.

The three speed evolved in to the four speeds with the addition of the over drive assembly.

You have to cut up the torsion bar x member, bang on the floor board & make a custom x member.

I put a 518 behind the Hemi last year. There is room for the 518s and there are plenty of them. The cores cost about the same and there is not a lot of money difference to rebuild each, so there is not much incentive for you to go with the lighter 500.

I did not have to do any crossmember cutting putting one into my 70 Cuda. There was one place that needed a 2 inch or so dent make in the floor board for clearance. I bought a fabbed crossmember, but they are not complicated to make. I am just not good at fabrication, so buying one already made was easier.

You will likely have the thing rebuilt before you put it in, so which one you start with (46, 47, 48) won't matter much in the end. As long as it's an RH (not RE) which had the small oval 2 or 3 pin connector.

The earliest 518s had the 2 pin connector. After a couple of years, they added the lock-up torque converter and the connector became a 3 pin to accommodate it.

I went with the non lock-up because there is a much better selection of torque converters. But for a milder street car, that might not be an issue and there can certainly be an argument made for the lockup. And they made the lock-up version longer, so there actually should be more of them.

If you have a competent 727 tranny guy locally, I feel the best bet is to grab a core at a salvage yard ($200+/-) and have it built the way you want it. That can be anywhere from $600 to $1,200.

Based on my experience, the cost of the swap could be anywhere from $1,000 to $2,500.

There are a number of threads around that detail the swap. One has step by step pictures. It is bit involved, but not too bad.

The 42RH is a smaller case and uses less HP to drive it. Problem is in stock config it isn't very strong. You can build it to handle the power. It just costs more. a stock 375hp 440 is easily doable with a hopped up 42RH. Another nice thing about the 42 is the better gear spread, first gear is a 2.74 vs. 2.45 first gear. Second is 1.54 vs. 1.45. OD is same on both at .69. I am building one to go behind a small block that should have about 400hp. SMR is a great company to get parts/info. Also PATC has parts and reasonable prices on torque converters, I just ordered mine from them.

Here is a great thread on the whole install and comments/questions...and of course wiring it up. GDonovan here has a great schematic too. http://www.cuda-challenger.com/cc/index.php?topic=84774.0

A 440 doesn't need a lower first gear than 2.45, so that point is moot.

I have a 518 non lockup for sale if you need something.

No, its not. Maybe he wants to run a taller gear in the back. If he wants to get to the 10:1 ratio, a 2.74 only needs a little more than a 3.55 gear where as a 2.45 needs a 4.10.

Yes, you can buy the gear set for a 46RH to get you the taller first gear, but it is quite expensive.

My old M.W. stocker like the Mopar 2.77 low gear set at the drags
That car ran low elevens back in the late 1980s on nine inch tires, that car liked the low gear set a lot. It out ran every Mopar M.W stocker in B/SA at the 1988 NHRA World finals
I had heard of several other M.W. racers in 1964 cars that said that same gear set in their cars spun the tires But they didn't change the rear gear ratio like I did when I did that gear swap in the tranny
If you don't test, you don't learn Some times you have to go sideways or backup to find what does work in testing

no cutting on an E body might be true, not so on my 1968 Charger.

The 904 evolved into the 500 which was later renamed the 42RH. The 727->518->46RH. 1996+ went to the RE models which are useless to you so make sure you don't get one of those.

Last time I checked, which was many years ago, the torque converter selection for the lockup units was poor. The non lockup units can use a 727 converter but I have read a loose converter with overdrive can have slip problems at cruise. I have read the 4sp's can be made to fit under a e body/71+ b body without cutting the crossmember with some grinding to the trans and careful fitment. I have read others who went to the GM 4 speed autos for better/cheaper aftermarket support and a smaller physical size to not have to cut. I ended up going with a stick because I don't like autos anyway.

PATC will make you a lock up converter, so you can get the stall you need. I have them making me about a 2500 stall lock up for the 42RH. Prices are good too.

Sorry, this would be going into a 1968 Charger. Thanks to everybody who commented. There is no substitute for this type of spirited discussion with a multitude of opinions and real world experience.

One day when I get off of my ass, I want to get one of those A41 4 speed kits from Silver Sport. They are based on the GM 700R4.

One of my local customers did that conversion, took out a good 727 and put in a GM 250 4 speed over drive, lots of money and problems making it work and shift the way he wants it to. Two different converters and lots of labor so far and he is still not happy
This went into a 1963 M.W. Fury with 3.23 gears, he said originally he wanted better mileage, it is a real nice drive to show car and doesn't get driven over 3000 miles a year. He later change the rear gear ratio from 3.23 to 3.55 to make it accelerate better
The parts and pieces cost a bunch over what he will save in fuel cost if he doesn't start driving it over 30,000 a year
My message is be careful of what you do to your car to make it better

Wise decision, the 700R4 is superior in all the areas discussed.