I don't know what manifolds you have, as the only mainfolds that were different for RHD/LHD are those found on the '49-52 8BA engine. The Manifolds used on CMP's are 78-9430 (R bank) and 78-9431 (L bank), which are the same for LHD/RHD trucks from 1937 to '46. In 1942, cars used the 21A-9430 on the Right Bank which had the studs "twisted" at an angle and the overall height is taller than the 78-9430, but there is no reason why you couldn't use this manifold provided you made up your engine pipe up to suit.
Maybe you are thinking engine pipes, which varied according to application?
The pics below show the manifolds used on CMP's and a comparison between a 78 and a 21A Right bank manifold. Sorry the pics aren't really clear, might redo them in daylight!
