I agree this sounds more like a tuning issue than the converter. As mentioned I would be checking the choke and idle mixtures on the carburetor. It also sounds like the engine may need some tuning on the timing curve. Depending on where the engine is idling and when the curve starts to come in, dropping down to idle may be altering the ignition lead enough to cause this behavior.
As far as the rough input into drive and reverse, did the TH400 build include the addition of extra friction material? This is common, but to do so typically coincides with removal of the wave plates in the clutch packs. Removing those will cause the transmission to jerk fairly heavily going into drive or reverse.
In regards to the torque converter, it sounds like you may be talking about a TSP converter. While I don't believe that is actually your issue, I had a TSP 9.5" spec'd for my combination in my TH400 and it was a great converter. Drove plenty positively under low load and flashed right to where I needed it under throttle. I opted to do a TSP in my 4l80e swap as well. I don't have any real drive time on that yet though.
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-Jason
1969 Pontiac Firebird
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