Brian,

A length that worked for someone else may not work for you. Considerations include intake manifolds, height of the engine relaative to the frame, and setback relative to the firewall.

If I were you, I'd set the engine in place and attach a manifold. Then I would get a dowell that fits the U-joint, mount that U-joint on the Vega input shaft and jockey it around to see where it should meet the firewall without interference. From that, you should be able to determine the protrusion of the column from the firewall. Then, I'd get a dowel and a pie-plate and simulate the steering wheel location in the cab with the seats in place. Start with a dowel that's too long, and chop it off until you're comfortable. Those two distances will solve your problem. That's how Ididit.