Is the engine in the car? Is so, I would install a stock system in two pieces. I would put a split pin into the shaft on the carburetor side of linkage with a clamp after you have fished the gas pedal half through the hole and around the back of the engine.
If the engine is out, just push the whole unit in through gas pedal the hole in the firewall from the engine bay.
If the engine is out, just push the whole unit in through gas pedal the hole in the firewall from the engine bay.