I will offer a counterpoint here at my own peril.
From the type of motor you are building I will assume ( you know what they say about assumption ) you are looking to run 10 flat and mid 9's.
Neal and Jason are both using 2004Rs with a Precision lock up converter.
Their transmissions are holding up to the beating and Neal has run a best of approximately 10.20 ( Neal jump in here and correct me if I am wrong.)
My Dad has a similar combo, he is using a 72 turbo, TH 400 with an Art Carr 9 inch non lock converter. Now Dad just changed the stall on his converter because it was slipping so bad that he was bouncing off of the rev limiter at half track. His new stall is a 3200, builds about 4 psi at 3500 and now the car pulls steadily thoughout the run. We will find out for sure tomorrow night when he takes it to Palmdale. The best time he has run so far is a 10.50 with some problems( burnt pistons you know the usual bugs

)
So there are two schools of thought here. I think it would depend on how the car will be used.
If you are using this on the street primarily and race on the weekends, I would use a lock up converter. Now from those who build these things, they will tell you to use all the billet pieces for the trans and it will hold up to 9.50 in the quarter.
If this will be a car that you trailer in and it is a race car, you may want to try a TH 400 and a non lock converter. Dads car does not use a trans brake and it is very responsive with the stall speed he has now.
One more thougt here too. I am starting to believe that we Buick guys get a little carried away with stall speeds on the converters. I think that a stall speed for your combo of around 3200 to 3500 at the most will work nicely and not have a lot of slippage. Any more than that is just a waste. Of course this just my 2 cents.
Good luck