What is the steering shaft diameter on a 67 chevelle? I think it is 3/4 by 36 but not sure. Beware, don't put a universal u-joint vs a rag joint. You will feel every small bump in the road big time in the steering. Sometimes it scares the you know what out of you. I got to go back to the rag joint.

There is a vibration dampning u-joint available from Borgeson--Ididit etc

I have the dampner joint down near the steering box and a u-joint near the column on my 59----
Can't feel a thing

I have expressed concern about using a universal joint in place of a flexible coupling in vehicles where the steering column directly attached to the steering gear. In this case the universal joint is unforgiving and has virtually no compliance side to side of axially. So there is the potential for very high loads on the lower steering column bearing. This is why General Motors always used a flexible coupling and created a Mandatory Steering Column Installation Procedure for these applications.

In the case of Chevelles (after 1967) and Gen II and later Camaros there is an intermediate steering shaft in between the steering column and the steering gear. This I-shaft had a pot coupling at the steering column end and a flexible coupling at the gear. The I-shaft eliminates any possiblilty of side loading the steering column lower bearing.

So in the case of a steering system with an I-shaft, I see no engineering reason why a metal universal joint can't be used. Two cautions however. The rubber disc in a flexible coupling isolates hydraulic noises that are generated in a power steering gear valve from being transmitted directly up the steering column and into the driver compartment. Also the rubber disc isolates individual road shocks from being transmitted into the steering wheel. You may not like these two results of having a solid universal joint.

Jim

Make sure the bearing at the top of the shaft isn't loose. This happened to my '69 Chevelle with a Flaming Rivers U-Joint set up and it was scary. We then installed a spacer to keep the bearing in place and it's fixed.

That said, you are going to get more road noise with a U-joint. But the steering is instantly responsive.