Flaming River coupler won't work with power steering box?

I'm converting the wifegod's 71 R/T Challenger from manual to power steering.
I picked up a new Mopar universal steering coupler from Flaming River which I hope to use with the new PS setup.

I'm researching the companies the sell upgraded rebuilt power steering gear boxes like Firm Feel, Steer and Gear, etc.

In talking to one vendor I was told I can't use the Flaming River coupler with a Mopar power steering box as the box's input gear has to flex in and out with the different power steering fluid pressures and the FR coupler is solid and won't flex?
He told me this isn't an issue with the manual steering boxes.

I called Flaming river and their confused tech guy wasn't much help.

Anyone have any information or experience?

Thanks in advance.

I agree with what you were told, what's wrong with the stock coupler?

Current one is worn out.

I've used them on two Mopars with ZERO problems after thousands of miles.

He's a salesman.

What do they know, he's FOS as to why you need movement in the coupling.

Call Firm Feel or Steer and Gear and get their opinion, they build the boxes they know.

One of the 2 companies mentioned is the one who told me.

I'm sure you know you can put new shoes and spring in them. Also make sure you have the shoes in right, otherwise there will be slop and some folks think that means wore out?

In a race car or a very ridged chassis I wouldn't expect much flex, but these old cars in close to stock form flex like crazy and then the coupler has to move, imo. Maybe that's why they are saying that?

Talk both with Bergman Auto Craft and Borgeson Steering.

Modern steering boxes have their advantages!

Agree the chassis is going to be moving around even if the gear shaft does not... I would want some give in the coupling to avoid beating up both the gear and and the column. I'd guess the light bearings and plastic bolts in the column would be sensitive more so than the gear itself.