Has anyone gone from manual to auto?
Currently planning on building a 1970 Challenger, 340 four speed. Due to hip arthritis issues, I currently can't operate a clutch. (plus I need to weld the z-bar frame bracket in, and I can't weld.) I plan to install all the clutch hardware for the future, but for now would like to be able to drive an automatic. The problem is I have none of the linkages or hardware for an auto trans. Is there a "drop-in" auto trans available for this situation?
Thanks!

I would source a 904. You'd either need a floor shifter or a steering column from an automatic & the rod/linkages to connect from the column
down to the trans.

The answer would be no.

There is no drop in automatic that will fix the problem of not having any of the linkages or hardware for an auto trans.

As for the 904, pretty sure there is a reason Chrysler never put one of those behind a 340.

A 727 was what Chrysler put behind a 340 is you went automatic. That would probably be the easiest to swap in place as all you would have to do is use the factory engineered setup for a painfree swap, well maybe not painfree to the wallet.

Only difficulty in the swap is making it factory-correct. If that's not an issue, then:
1. SB 727 or 904 & converter & mounting plate & dipstick assy, depending on your engine's power level.
2. Floor shifter - I like the Cheetah SCS, others like B&M or whatever; cable actuation means no factory linkage required. I've mounted a Cheetah on a 1/4" aluminum plate secured to the floor using the 4-speed boot screw holes.
3. Trans cooler & lines - stick an aftermarket cooler up front, run whatever hard lines you like, with short rubber hoses.
4. Driveshaft length may need to be addressed; you might want a 2nd driveshaft for this if you want to retain the 4-speed stuff for later.
5. I don't recall a trans-mount issue on my swap, but I bought a 727 mount for it anyway because the stick one was 50 years old.

Don't forget kick down linkage or cable set up.

In an E Body the driveshaft is the same length between 4 speed and 727, 904 takes a longer shaft than the 727 with a corresponding slip yolk made for the smaller output shaft.

Using an aftermarket cable shifter makes the swap easier by avoiding the need for a column change or the mounting brackets for the shifter.

Cooler lines are not a issue - purchase them or bend your own.

Using a late style 998/999 (some guys call it a 904 - 998/999 has wider front bands and more clutch discs) gets you the 2.7 low gear set. It can easily handle the power of most 340’s if properly built.

What year do 998/999 transmissions start?

I believe around 1973 The designations were modified for plant use.

Don’t worry about the trans designation, most all “baby 727’s” found in truck and V-8 car applications had at a minimum a 4 disc front clutch, some had the 5 disc.

I would look for one from 1980 or later in order to get one with the low gear set.
Many will have lock up - you can easily use a lockup converter and trans behind a warmed up small block without worrying about the converter. Finding one with PTU (part throttle unlock) allows you to control lockup electrically. I wouldn’t hot rod a production lockup converter unless it is a real late (1999 or later ) converter.

If you choose to use a high stall aftermarket converter then you’ll have to change the input shaft and converter.