Not necessarily. I have a friend in Orange CA, who is an Aussie and is in the business of sending stuff back to Australia. His father runs the business this end. They are always bringing containers of cars down here and are looking for stuff to fit in around the cars. How close are you to Orange??

I have located a cast iron, RH clutch fork bellhousing over here out of a 6cyl Chev truck, with side mounts on the bellhousing. The bellhousing pattern is the same as a SBC, so this is probably off a '62 as you describe?
As for the mid '60's all synchro box, will it fit straight in where the '57 box does? Note that the RHD conversion has a big rat-trap arrangement that bolts under the box and operates the shift levers on the LH side of the box. What year model did these originate in? Is it the 3-speed Saginaw you are talking about? I also think my mate has a 3-speed T10 in his shed (it has T16 on the side I think).

Byron