The thing about the MGC manual transmission that's different from the MGB is the outside case....
Totally different...you're talking changing from a flywheel to a flex plate (or vice versa), changing shifting mechansm, changing pedals...
I think you have an automatic MGC - that's a rare bird (I've got a '69 automatic MGC)....were it me, I'd find another car that already has the transmission I wanted...that car's too valuable to whack up, IMHO!