A vehicle built before 1 January, 1979, will qualify for historic road tax from April 2019 - unless it has been substantially altered. Unfortunately, what constitutes 'substantially altered' is currently a bit of a grey area. The definition currently says that to be considered as a vehicle of historic interest (VHI) the following components need to be of a design of which would have been fitted to that vehicle at the time of its manufacture:
• the original unmodified chassis or body shell (including any sub frames) or,
• a new chassis or monocoque bodyshell (including any subframes) of the same specification as the
original
• suspension (front and back)
• steering assembly
• all axles
• transmission
• engine
An MGB GT that's been converted to a V8, could potentially have had alterations to those components. However, because the factory produced its own V8s it should qualify as a VHI.