Jumping on the band wagon, with a few words to support headers and the Peco one in particular.
Lovely to hear from you Roger, but even though the factory staff may talk a bit funny, the Peco headers were originally designed to fit Left Hand Drive cars. Like many aging product designs, they used to fit just fine, but many bent and welded exhaust components have drifted over the years. I have installed many over the years without steering shaft conflict, though sometimes on late model cars, I've had to remove the manifold studs to slide the header into place.
The problem is that companies supplying parts like this have moved on to more modern vehicles, and I swear based on the fit issues I see they have the guy that sweeps the floor knock out the orders for the old cars stuff.
A major problem I've also run into is that the manufacturers no longer have access to the cars to check fit their products. We ran into the same problem with Falcon stainless exhausts, we would have to fit the component to a car, modify it, then ship the unit back to the UK to try to get any changes made.
It's not just the UK manufacturers. I've had an ongoing argument with Pacesetter on their Monza exhaust for the early MGB. I compared a new old stock system from 1984 that came with a car I bought to the current manufacture and they both have the same major fit problem. After not getting anywhere, I finally asked to have Moss no longer carry the early version of the Monza exhaust. It's a pity, as it looks and sounds nice once fitted. What's really frustrating is that the rubber bumper version fits without a problem! Headers do have their place. As a child, I was constantly frustrated by blown manifold to down pipe gaskets, swapping to headers solved that issue and I just had to be careful with fitting the intake/exhaust gasket. I continue to use headers on my more performance oriented vehicles, but understand that fitting them and keeping gaskets working can be a royal pain.
I agree that the standard cast iron manifold is a great design. But, it is getting difficult to find good down pipes, for the same reasons the headers have problems. Usually the stainless steel ones fit well, but any mild steel components for the car tend to be poor. See comment about the floor sweeper.
This is why Moss used a simpler design headpipe on the Sport Exhaust, that uses flat gaskets and why we designed our own stock Touring system. A lot of work went into ensuring the head pipe of the Touring system fit well and we keep a set of jigs for them in California to ensure they stay that way.
Happily both Bell and Falcon are also currently making good stainless head pipes, so there is hope for the world.
My recommendation for the best street system is in line with the other great minds on the list. Use a cast iron manifold to match the intake manifold used. I'm hearing really good things about Jet Hot coating the inside and outside. New studs, new brass nuts and a good quality down pipe.
Circling back to the original question. The Peco header should fit without steering shaft clearance issues. If there is a conflict, I would recommend trying to get it replaced by the supplier. Failing that, the middle pipe can be dented enough to clear the shaft. There was no mention of the model year, but my guess is this is a rubber bumper car and shimming the left hand motor mount can also fix this fit issue.
Unless one has access to a mill, absolutely do not try to modify the flange. If you want to get clearance by moving the header, install it and apply a lever to the bottom pipe. It will move.
Moss Motors Tech