That's about the same place my upper U-joint ended up at.
Approx 1/4" + between frame rail and fat part of the U-joint.

Couple of things you coud do is to substitute a longer junkyard GM lower shaft - if you could adapt it to your tilt mechanism.
Fwiw - the GM steering shafts are 3/4" OD and 36 splines on the lower end.
Steering wheel splines are the same for many years of GM production.

Another way out is to replace the Vega box with one from the next size up cars.
Harwood used to set their 32 frames up for these El Camino and other similar models slightly larger steering boxes that bolt right onto the Vega steering box plates.
Nice part about the next size up box - and they're barely larger - is the input shaft sticks out to the rear quite a ways further than does the stock Vega.
Looking from here, the extended input shaft boxes have close to the same length on the input shaft you've mocked up.

Not sure what you'd find, but an Internet search for Saginaw steering boxes may net you the number to look for.

I see you've dropped in on the RRT.
The people there know what the box number is if you care to post a question.

Enjenjo on the RRT is a national treasure as far as being a fount of knowledge goes.
You couldn't go far wrong in asking him....