Hey Bob.. Just got your email and thought that I too better pop-in and say hello!
The lower pics in the above post are of my subframe conversion, and I am using a Saginaw 800 series box, also out of an Australian delivery Pontiac. It's an identical mirror copy of the LHD Saginaw boxes (God bless Saginaw!
).
The problem that I can see with using a Holden or Ford Falcon box (which I hear is quite common) or any other, is where exactly to locate it. The Fore/aft, side, up/down and tilt angle can be a bit of guess work. You would want to hope that while the height of the Pitman arm is allowing the drag link to be at the correct height also, the steering column is still able to couple with it too. What is the height of the fitted Pitman arm in relation to the steering input shaft compared to the Saginaw?
With the Saginaw, once the centerline of the subframe is established, the LHD mounting holes are simply transfered across. A reasonable amount of fabrication is required if you want it to look right, but also because the rails are a 1/2" different distance from the chassis centerline. I also used a portion of Australian delvery Pontiac subframe from the formed piece that sits behind the steering box. The jig allowed a rigid foundation and a high level of accuracy in doing the conversion. The idler side was exactly the same process.
Using the RH Saginaw and mounting it that way, I have as much room as any normal LHD Camaro. I too have a big block and the headers aren't such a squeeze. Custom made is mandatory and as for manifolds, I'm not sure. Often LH & RH were different so, only one way to find that out!
I also needed to make up a jig for the draglink. I didn't weld it though, I had Mr Blue (a qualified welder & chassis guru do it) and then had it Xrayed heat treated.. All that cost me $600 but..
The heater box was a lot of fun. Lot's of work, nearly lost a finger!!, but the end result is very convincing.
There is a RHD fiberglass on available down here, but the finish and lack of detail was just not what I wanted. (Good value at $130 though.)
Any details you need to know, just ask me right here or email me.