You can omit the center seal, plug this passage, enlarge the 2nd and 3rd feed holes in the stock seperator plate and you will have greatly increased the torque capacity of a stock TH350 as well as gained a firmer shift.
Even firmer shifts can be accomplished with checkball removal and modifications to the accumulators. I recommend NOT modifying the 1-2 accumulator. Retain the cushion spring and do not block the circuit. The 2-3 accumulator can be blocked by several methods.
Removing the 2-3 accumulator spring and using a spacer (nut, washers, cut down section of tubing) between the piston and the e-clip is the "shadetree' method. You can also block the feed in the valve body to the 2-3 accumulator.