Has anyone swapped out the standard 3 speed TH350 automatic transmission for a 4 speed automatic transmission to get an overdrive final gear.  My 79 turns about 2900/3000 rpms at 65 MPH which is a lot of stress for long distances.

If so, what transmission did you go with? Did you change drive shaft length? Transmission mount? Can you keep the 3 speed auto shifter?

Thanks

Nope, Never continued with the project. A multitude of things has to be done. The 700r4 is to long. The 2004r is better. Driveshaft, cross member, shifter, center console, TV cable, and other things that can go wrong, Will go wrong. It will NEVER pay for itself in savings on fuel. There are kits that make it possible. They are not cheap. Better to drive slower, and enjoy the scenery. Take the back roads, and let the Big Dogs bark in the Fast Lanes. I moved a lot of money just preparing. 

LOL, sounds like good advise.

I haven't done it, but I do know a few chaps over here, in the UK and in France, who have, or have had it done.  (Corvette Kingdom in the UK used to do a few and almost had a "kit" 'of the various parts needed.) 

Thos who have made the swap have noticed big improvements in drivability, with the rpm at a 70mph cruising speed dropping to round 2000, which must make life more pleasant! 😎 And I guess, there has to be fuel savings.   

I did fancy it and considered the change when we were still living in the UK, but never got around to it.  I can't deny that the reduced RPMs would be welcome now as we do more long distance trips on good Autoroutes.   But definately not a job I would consider attempting myself!😄

I replaced my TH400 with a built 200-4R trans many years ago. I did purchase the whole kit from one of the companies in California so it wasn’t cheap, $1800 at the time.

the biggest reason for the change was my casting on the TH400 was very bad and I could not get it to stop leaking trans fluid at the pan. 
plus I did not like 3600 rpm’s at 70mph.

I have a 3.55 rear gear.
Now I can go anywhere and I get around 17mpg where I was only getting 11mpg. 
that’s not going to pay for itself in gas savings but it is less wear and tear on the rest of the drivetrain.

I did the swap Richard, of a '73 L48's TH400 to a rebuilt M-20. Got parts from here and there. Transmission crossmember, pedal rack, shifter end on the frame weld, tunnel shift hole mod, new propeller shaft, just to name a few. Did it all by myself, and got the pictures to prove it. A little bit each day, and 6 months later I was having fun driving an honest sports car. Was 68 at the time, with a bad hip too.☺️

Standard 3.08 rear end, 225/70R15s, and at 2500rpm, it's turning 68 mph. Not at all today's standard. Extra $$ at the pumps for sure. But let's remind ourselves folks; we didn't buy these cars for fuel economy!😎

Kept all the TH400 stuff, as it's #s are on the build sheet. Kept original block too.

All in all, I'm very glad I did it. 

Steve 

I did the swap Richard, of a '73 L48's TH400 to a rebuilt M-20. Got parts from here and there. Transmission crossmember, pedal rack, shifter end on the frame weld, tunnel shift hole mod, new propeller shaft, just to name a few. Did it all by myself, and got the pictures to prove it. A little bit each day, and 6 months later I was having fun driving an honest sports car. Was 68 at the time, with a bad hip too.☺️

Standard 3.08 rear end, 225/70R15s, and at 2500rpm, it's turning 68 mph. Not at all today's standard. Extra $$ at the pumps for sure. But let's remind ourselves folks; we didn't buy these cars for fuel economy!😎

Kept all the TH400 stuff, as it's #s are on the build sheet. Kept original block too.

All in all, I'm very glad I did it. 

Steve 

He doesn't want to go to a manual tranny.  😕

🥴

Steve