I used a reversed Corvair box on mine. I bought a good used one and performed the operation to flip it myself. They are extremely compact and still really strong. Most of the parts are still available and they go pretty cheap if you're patient. You can also buy new aftermarket copies.
The process of reversing it is easy and if you don't feel competent to do it yourself, any machine shop can do it for you.
Other options are the Vega, the Mustang/Pinto rack and pinion, and numerous other factory and aftermarket options.
You could also reseal your sprint car box if parts are still available.