Welcome Mick
A couple of observations:
As others have suggested the 711M block is the optimum one to go for. That is not to say the 2737 block is a weak engine , however its not as tunable with its chambered cylinder head and weaker big end caps
Changing the engine to Vauxhall, Mazda etc will mean that the car will no longer qualify as a historic vehicle by my reading of the regulations as its a substantial change so you will need an MoT each year. I don't think will affect the ' free' road tax provision though.
Arwyn
