The steering box is the obvious bit, but beware, the likely source (TR3) has a longer steering column. Given the rarity of spare Peerless steering boxes you might feel that a rack conversion would be a better idea and it would certainly be cheaper than getting a TR3 one shortened, even if you could find anyone to do it. A rack also does away with the idler the other side, but they wear like the very devil, so you'll be glad to loose that. Commercial Rack conversion kits were made, or contact Eric De Vries, who has done a RHD to LHD conversion.
Moving the pedal box over is not insurmountable, it will probably be easier to make a new mounting plate than try to cut the old one off, but remember that this is serious safety-critical welding. I suspect that you might have to lift (or at least cut away) the dash/body to be able to do the welding
You'll need to re-plumb the brake lines and re-fabricate the trrottle, and a TR2/3 throttle pedal would do. You will also need a RHD instrument panel, but this should be pretty easy to fabricate by moulding off the reverse of the LHD one that you've got. And the wiring harness will need replacing/modifying because all the instruments are the other side.
And finally.... Only the drivers door is lockable.
Good Luck
FF
Most of the above explains what the Warwick was all about!
I'm going to convert my car the other way, from rhd to lhd. We could swap parts if you like. Especially the instrument panel could be easier to swap than convert. I live in Sweden but I have friends who regularily goes to britain. You could email me direct to: marcus.andang@gmail.com