IIRC a safe bolt-in for the TH350 is the Vega converter, will add a few hundred stall.
Before you spend any more money on the trans consider a 200-4R.
If you haven't already done this: you should drop at least 5° from your advance curve and add it to your initial spark setting - this will pick up your response in 1st a lot, and raise your stall speed slightly. Listen for pinging, add more if OK.
Have you tried advancing the cam 4°?
