GM 5 speed is the way to go, the shifter is out the top so you don't need any room for levers. Also the hydraulic clutch keeps you from having to work around the clutch equalizer and rods. One problem is the shifter lever is a little short for a older car with a high bench seat. You can order clutches and discs for later model sbc. You'll love the 5th gear it will be the best gas mileage that car will ever get.
good luck