the only gearboxes you can use are K50 from an ae85 (or another car, but must fit A series motor) or a T50 with an A bellhousing (which will cost you an arm and a leg)
you have one other option which is to get a niteparts W5* gearbox bellhousing (custom) and a w series gearbox... each have theyre strong points.. k50 is cheaper easier to find, cable clutch but its weak...
T50 is stronger but getting hard to find and you'll need to link up a hydro clutch.
w5* are strong as but more expensive and you'll probly need a custom box cross member and drive shaft, might also need to take a ding out of the firewall/box tunnel too but im not sure
hope it helps.. if you want more info PM me ive 4age'd 2 dx's so far