Torque converter for 340

I tell you what, it really depends on a bunch of stuff; like;
application,application,and mostly,application,
But also on application,and application,
If you fail to take into account the application,
Then in your application,any application may not be a good fit.
Also if your engine is tired or worn out or just never made much low-rpm torque, then any guess is just a guess.
Plus if you have 3.91s or some other HO rear gear, you can get away with less TC..... or vice versa;
if you have hiway gears, the tired 340 might want a lil more TC.
But if your 340 is an animal at low rpm then the TC will depend on other factors, mostly like.....application,and application.
Without an application forthcoming, Poster #2 nailed and, and poster #3 cleared up the stock application, which, with a stock, not-tired 340, is also bang-on;
and I offer the range of 2400 to 3800, depending on application, which will be correct for certain applications.
PS, I like a 2800 minimum; with 3.23s as a minimum, if you do much/significant hiway traveling;
or 3.55s as a combination machine,
or 3.73s if you like to show off a lil.
For a city-car, and a lil more and better tire frying ability try 3.91s with a 3000.
But if you're looking for fuel-economy, swap out the 340cam for a teener-type cam, install a 2bbl, and 2.73s and then the factory 2250TC will be pretty sweet, for this application.
I don't know about the racing applications, but I have heard that hi-reving 340s can like 4.30s or 4.56s depending on the camming. Of course then in this application the 340 might like a 3800 to 4500, or some other TC depending on what rear is being run and the trap-speed; it is very application-specific.