Converting a 3 speed manual to 4

I just did this conversion on my '72 Duster 225. Look up Brewers Performance. They are a company out of Ohio who only deal with A-833s, and they were recommended to me by the fine folks here at FABO. The entire kit I got cost just over two grand, and it included tranny, bell, clutch fork, Hurst shifter kit, and a gallon of gear oil. Everything bolted right up and I haven't looked back. As far as the gear ratios, if you get the non-overdive A-833, it will be all four gears covering the spread of the original three. First gear will be the same and fourth will be the same as the current third, but having two gears between first and the 1:1 makes a huge difference. If memory serves me correct the three-speed ratios were: 1st 3.09:1, 2nd 1.70:1, and 3rd 1:1. The four-speed for the sixes is: 1st 3.09:1, 2nd 1.92:1, 3rd 1.40:1, and 4th 1:1. If you get an OD A-833, first through third will be the same with a .73:1 fourth gear.