Finetuning questions - do I need a high-stall torque converter ?

Considering where you are and that you likely want to be able to cruise it on the autobahn I would leave the gears as they are.

A good torque converter is a viable option just make sure you use an external trans cooler.3000 rpm would suit you well maybe 3500 if it is efficient.

A 600/650 vacuum secondary carb would help the upper mid range through the top end noticeably.

Your step headers are a bit big for this engine but I wouldn't change them out unless you have some 1 5/8 long tubes lying around.

You mentioned a lot of wheelspin on your second pass.Seems there is no problem doing a burnout.

I would experiment with shift points from 5500-5800 rpm to find the best E.T.