I'm 6'4" as well, and have a 36" inseam. After much research and reading other people's posts, I tried the Windstrom Manta: extra-tall model with the "TS" or Top Spoiler option. I think I hit the proverbial home run.
To see pictures of my set up, go here:
After reading other's experiences with various windscreens, I believe this is about the best you could hope for. I'm quite satisfied with this set-up. I have a 3/4 helmet, the HJC CL-33. I ride with my face shield in the up position almost all the time now - and I experience almost no buffeting on the helmet, except in the most windy conditions. Even then, it's not objectionable. I get very little wind in the face and my shield never spontaneously closes or gets blown around.
I do get some buffeting on my shoulders, but what the heck, it is a motorcycle. I have experimented with some pieces of plexiglass by holding them in just to the inside of my mirrors, and it virtually eliminates the buffeting on my shoulders, but then I have a strange low-pitched rumble and my head vibrates in a weird way.
I have ridden extensively without any windscreen whatsoever and this is also a very good option as the clean blast of air is just that - clean. No buffeting whatsoever. If not for bugs, I would be happy with this option. As far as your attempt to block just the air hitting your chest - I don't believe you'll ever be able to achieve this as I tried cutting my stock shield down in many different configurations before deciding it wouldn't work.
I do have the Madstad windshield mount. It's a good product to experiement with different windscreen positions, but in the end, it did not help me as I found the best position for the Manta could be achieved without the Madstad. Also, with a windscreen as tall as the Manta, the Madstad will not under any circumstances keep the windscreen in place no matter how hard you tighten the quick adjustment thumbscrews, so don't even bother with them. Use regular nuts/bolts instead.
Oh, and the Manta is butt-ugly, but I'm not obsessed with aesthetics, so it doesn't bother me. If you were in NY, you could try my bike before buying a windscreen. Let me know if you have any additional questions. It's a big financial commitment, you want to make the right choice the first time.