In the absence of JR who is probably on holiday as usual,
I think the App K FIA period for the MGB for historic racing was 1962 to 1965 thus the post 1975 rubber bumper version, no matter how you build it will not be suitable. Likewise I think there is a
post historic rallying class but I'm not sure what dates that covers, however I suspect the heavier rubber bumper chassis may be unsuitable.
That said you can build one to the spec you suggest and run it in modified classes and to that end I recommend you contact the MG Car Club
www.mgcc.com. There's loads of people there who would be delighted to assist.
Welcome to our friendly forum and let us know how you get on.