Power to Manual conversion questions

You can use a factory manual brake pushrod. It does not need to be adjustable.
I don't know the parts availability in the UK, but see if you can get a rebuilt or new power brake MC for a 73-75 disc brake A body but run it MANUALLY. 7/8" bore. It's all a direct bolt in.
You may have to buy new pre-bent brake lines, bend and flare new ones from scratch, or rebend the ones you have.
There is a white foam gasket that goes between the MC and firewall. They are reproduced or make your own.
I predict that after driving the car this way you won't go back to your power brake setup.
There really is no reason to run power brakes on an A body unless you are physically limited or are seeking a 100% correct restoration.