From past experience, try to use as many parts from a common gearbox. If you are looking for a shifting gearbox look at
http://www.andymark.com/product-p/am-0114.htm. The super shifter is on the slightly heavy side. This season, my team built custom gearboxes, we repackaged the majority of the Super Shifter parts into a lighter/more compact gearbox.
Hope this helps