A couple of things you might need to think about.
Before you make the decision on a 55/56 or 57 dash, consider the column requirements for each dash.
Column: the 55/56 dash uses the same year column. The design allows you to shorten the column a bit if you want to, or need to. The 57 dash needs the 57 column. The dash cluster restricts any shortening. If you want an aftermarket tilt column, the tilt part pushes the steering wheel a bit closer to you. Can be worked around a few different ways - move the seat back, use a shallower steering wheel.
Instruments/cluster: you already have the 57 instruments. if you decide on a 56 dash, you will need to source all instruments and everything else related to the dash.
If you want an integral power steering box that will bolt straight in using the original mounting holes, then I have a Delphi 600 Series box that will do that. As the OP is converting to RHD, then holes will have to be drilled in the chassis anyhow. As well as the box, I have quality Borgeson components such as a steering shaft, column support, rag joint and a Saginaw pump kit to suit a SBC. My box won't work with a big block, unless the engine is moved to the left a few inches, and I believe that there is a kit available from a company in Castlemaine that can do this. I have developed this kit and have sold several to Tri-5 owners, plus I also have a kit for the '58-'64 Chevs and Pontiacs. Let me know if I can help.